apm: fix race in DefaultTracer

[axw]

Fix a race in DefaultTracer which could lead to two tracers being created, one of which would be closed.
We must hold the write lock while creating the default tracer, checking again that it was not set after releasing the read lock.

Fixes #1247

[marclop]

Thanks for the quick fix!

[apmmachine]

💚 Build Succeeded

the below badges are clickable and redirect to their specific view in the CI or DOCS

Expand to view the summary

Build stats

• Start Time: 2022-05-20T01:01:08.680+0000

• Duration: 41 min 27 sec

Test stats 🧪

Test	Results
Failed	0
Passed	7372
Skipped	174
Total	7546

🤖 GitHub comments

To re-run your PR in the CI, just comment with:

• /test : Re-trigger the build.

• run benchmark tests : Run the benchmark test.

• run elasticsearch-ci/docs : Re-trigger the docs validation. (use unformatted text in the comment!)

[apmmachine]

🌐 Coverage report

Name	Metrics % (covered/total)	Diff
Packages	100.0% (59/59)	💚
Files	99.346% (152/153)	❕
Classes	96.812% (334/345)	❕
Methods	90.35% (955/1057)	❕
Lines	82.621% (11063/13390)	❕
Conditionals	100.0% (0/0)	💚