apmsoak: Support arbitrary headers via -header

[marclop]

Motivation/summary

Adds support adding arbitrary headers to the apmsoak binary via a new header flag which can be set multiple times.

Checklist

- [ ] Update CHANGELOG.asciidoc
- [ ] Update package changelog.yml (only if changes to apmpackage have been made)
- [ ] Documentation has been updated

How to test these changes

N/A

[mergify]

This pull request does not have a backport label. Could you fix it @marclop? 🙏
To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-7.x is the label to automatically backport to the 7.x branch.
• backport-7./d is the label to automatically backport to the 7./d branch. /d is the digit

NOTE: backport-skip has been added to this pull request.

[apmmachine]

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• Start Time: 2023-03-29T08:04:21.316+0000

• Duration: 7 min 36 sec

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[apmmachine]

📚 Go benchmark report

Diff with the main branch

goos: linux
goarch: amd64
pkg: github.com/elastic/apm-server/internal/agentcfg
cpu: 12th Gen Intel(R) Core(TM) i5-12500
                                  │ build/main/bench.out │             bench.out              │
                                  │        sec/op        │   sec/op     vs base               │
FetchAndAdd/FetchFromCache-12                46.11n ± 0%   41.11n ± 0%  -10.85% (p=0.002 n=6)
FetchAndAdd/FetchAndAddToCache-12           104.20n ± 4%   93.72n ± 4%  -10.05% (p=0.002 n=6)
geomean                                      69.32n        62.07n       -10.45%

                                  │ build/main/bench.out │             bench.out              │
                                  │         B/op         │    B/op     vs base                │
geomean                                                ²               +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                                  │ build/main/bench.out │             bench.out              │
                                  │      allocs/op       │ allocs/op   vs base                │
geomean                                                ²               +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

pkg: github.com/elastic/apm-server/internal/beater/request
                                             │ build/main/bench.out │              bench.out              │
                                             │        sec/op        │    sec/op     vs base               │
ContextResetContentEncoding/empty-12                   136.1n ±  2%   122.4n ±  1%  -10.03% (p=0.002 n=6)
ContextResetContentEncoding/uncompressed-12            164.7n ±  1%   145.1n ±  2%  -11.87% (p=0.002 n=6)
ContextResetContentEncoding/deflate-12                 3.754µ ±  4%   3.969µ ±  7%   +5.73% (p=0.026 n=6)
geomean                                                933.7n         923.5n         -1.09%

                                             │ build/main/bench.out │              bench.out               │
                                             │         B/op         │     B/op      vs base                │
geomean                                                           ²                 +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

                                             │ build/main/bench.out │             bench.out              │
                                             │      allocs/op       │ allocs/op   vs base                │
geomean                                                           ²               +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

pkg: github.com/elastic/apm-server/internal/publish
             │ build/main/bench.out │            bench.out             │
             │        sec/op        │   sec/op    vs base              │
Publisher-12             1.003 ± 0%   1.004 ± 0%  +0.08% (p=0.004 n=6)

             │ build/main/bench.out │           bench.out            │
             │         B/op         │     B/op       vs base         │

             │ build/main/bench.out │           bench.out           │
             │      allocs/op       │  allocs/op    vs base         │

pkg: github.com/elastic/apm-server/x-pack/apm-server/aggregation/spanmetrics
                 │ build/main/bench.out │          bench.out           │
                 │        sec/op        │   sec/op     vs base         │

                 │ build/main/bench.out │            bench.out            │
                 │         B/op         │     B/op      vs base           │
¹ all samples are equal

                 │ build/main/bench.out │           bench.out           │
                 │      allocs/op       │ allocs/op   vs base           │
¹ all samples are equal

pkg: github.com/elastic/apm-server/x-pack/apm-server/aggregation/txmetrics
                        │ build/main/bench.out │          bench.out           │
                        │        sec/op        │   sec/op     vs base         │

                        │ build/main/bench.out │           bench.out           │
                        │         B/op         │    B/op     vs base           │
¹ all samples are equal

                        │ build/main/bench.out │           bench.out           │
                        │      allocs/op       │ allocs/op   vs base           │
¹ all samples are equal

pkg: github.com/elastic/apm-server/x-pack/apm-server/sampling
               │ build/main/bench.out │             bench.out              │
               │        sec/op        │    sec/op     vs base              │
geomean                  622.3n         569.8n        -8.44%

               │ build/main/bench.out │              bench.out               │
               │         B/op         │     B/op      vs base                │
Process-12             9.263Ki ± 3%     9.151Ki ± 1%  -1.20% (p=0.037 n=6)
geomean                             ²                 -0.60%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

               │ build/main/bench.out │             bench.out              │
               │      allocs/op       │ allocs/op   vs base                │
geomean                             ²               +0.00%               ²
¹ all samples are equal
² summaries must be >0 to compute geomean

pkg: github.com/elastic/apm-server/x-pack/apm-server/sampling/eventstorage
                                            │ build/main/bench.out │               bench.out               │
                                            │        sec/op        │     sec/op      vs base               │
ShardedWriteTransactionContended-12                   5.674µ ± 22%    9.470µ ±  42%  +66.92% (p=0.026 n=6)
WriteTransaction/json_codec-12                        9.386µ ± 79%    4.015µ ±  11%  -57.22% (p=0.002 n=6)
WriteTransaction/json_codec_big_tx-12                15.256µ ± 36%    4.890µ ±   4%  -67.95% (p=0.002 n=6)
ReadEvents/json_codec/0_events-12                     350.3n ±  8%    306.8n ±   4%  -12.41% (p=0.002 n=6)
ReadEvents/json_codec/1_events-12                     11.37µ ±  7%    10.78µ ±   4%   -5.17% (p=0.041 n=6)
ReadEvents/json_codec_big_tx/0_events-12              355.5n ±  8%    302.6n ±  13%  -14.87% (p=0.009 n=6)
ReadEvents/json_codec_big_tx/1_events-12              12.39µ ±  4%    11.49µ ±   4%   -7.26% (p=0.002 n=6)
ReadEvents/nop_codec/0_events-12                      350.9n ±  7%    310.2n ±   7%  -11.60% (p=0.004 n=6)
ReadEvents/nop_codec/10_events-12                     19.19µ ± 37%    30.59µ ±  18%  +59.35% (p=0.009 n=6)
ReadEvents/nop_codec_big_tx/0_events-12               343.4n ±  6%    304.2n ±  13%  -11.42% (p=0.015 n=6)
IsTraceSampled/sampled-12                             78.62n ±  3%    69.13n ±   2%  -12.07% (p=0.002 n=6)
IsTraceSampled/unsampled-12                           77.98n ±  1%    71.18n ±   7%   -8.71% (p=0.002 n=6)
IsTraceSampled/unknown-12                             416.8n ±  2%    378.4n ±   5%   -9.24% (p=0.002 n=6)
geomean                                               31.35µ          30.11µ          -3.95%

                                            │ build/main/bench.out │               bench.out                │
                                            │         B/op         │      B/op       vs base                │
WriteTransaction/json_codec_big_tx-12                3.688Ki ±  0%    3.686Ki ±  0%  -0.04% (p=0.022 n=6)
geomean                                              31.39Ki          31.55Ki        +0.52%
¹ all samples are equal

                                            │ build/main/bench.out │              bench.out              │
                                            │      allocs/op       │  allocs/op   vs base                │
geomean                                                 144.7         144.7       +0.00%
¹ all samples are equal

report generated with https://pkg.go.dev/golang.org/x/perf/cmd/benchstat