internal/apmlog: fix log levels

[axw]

• Add the "trace" level.
• In addition to "warn", we should also accept "warning".

See the spec: https://github.com/elastic/apm/blob/master/specs/agents/logging.md

(Thanks @trentm!)

[codecov-io]

Codecov Report

Merging #860 (6b53652) into master (491755f) will decrease coverage by 0.05%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #860      +/-   ##
==========================================
- Coverage   83.77%   83.71%   -0.06%     
==========================================
  Files         142      142              
  Lines        7913     7921       +8     
==========================================
+ Hits         6629     6631       +2     
- Misses        876      883       +7     
+ Partials      408      407       -1     

Impacted Files	Coverage Δ
internal/apmlog/logger.go	72.83% <0.00%> (-7.99%)	⬇️
tracer.go	89.06% <0.00%> (+0.31%)	⬆️

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Build stats

• Build Cause: Pull request #860 opened

• Start Time: 2020-12-14T02:47:33.454+0000

• Duration: 24 min 49 sec

Test stats 🧪

Test	Results
Failed	0
Passed	7663
Skipped	188
Total	7851