Host decorator

[camdencheek]

Description of Changes

Add a host metadata decorator. Supports adding hostname and ip address to the log entries.

Currently there is no customization for what IP address, but in the future, we may add options for preferred interface, ipv4 vs ipv6, or an option to add external IP.

I was unable to figure out a way to mock calls to net.Interfaces() effectively, so our tests currently don't test the IP functionality at all. Would love to hear ideas, but I'm currently convinced it's not possible. See this for more detail: https://stackoverflow.com/questions/24564840/golang-net-package-mocking

Please check that the PR fulfills these requirements

• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)
• Add a changelog entry (for non-trivial bug fixes / features)
• CI passes

[codecov]

Codecov Report

Merging #85 into master will decrease coverage by 0.48%.
The diff coverage is 38.89%.

@@            Coverage Diff             @@
##           master      #85      +/-   ##
==========================================
- Coverage   74.62%   74.14%   -0.48%     
==========================================
  Files          66       67       +1     
  Lines        3991     4045      +54     
==========================================
+ Hits         2978     2999      +21     
- Misses        762      790      +28     
- Partials      251      256       +5     

Impacted Files	Coverage Δ
operator/builtin/transformer/host_metadata.go	38.89% <38.89%> (ø)
operator/builtin/input/file/read_to_end.go	62.69% <0.00%> (ø)

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 6ec38be...149f66d. Read the comment docs.

[djaglowski]

Log Files	Logs / Second	CPU Avg (%)	CPU Avg Δ (%)	Memory Avg (MB)	Memory Avg Δ (MB)
1	1000	2.9483392	-0.4828403	27.368534	-1.1893864
1	5000	7.68986	-0.48279333	39.767647	-0.34819412
1	10000	14.379564	+0.01718998	52.864494	-0.5818939
1	50000	58.086323	+0.0039138794	1372.0466	+1202.7341
1	100000	106.64057	-1.9049911	890.1001	+606.68494
10	100	5.0173283	+0.2586112	30.231142	+2.5378494
10	500	10.759035	+0.7933445	41.40948	+3.890892
10	1000	16.448626	+0.2240963	56.39709	+3.9500275
10	5000	64.60442	+0.7589302	247.6184	+80.63617
10	10000	42.777935	-73.864334	2633.6519	+2316.222