feat(logging): forward grpc internal logging to zap

[GeorgeMac]

This continues on from a discussion in the original logrus to zap PR: #1020

This forwards all gRPC internal logging into zap.

Originally, I tried to use grpc_zap.ReplaceGrpcLoggerV2 in the first PR, which led to lots of gRPC chatter in the logs.
This was because the default zap level for all Flipt logs was INFO and gRPC has a lot of INFO chatter.

Normally, the default severity level for gRPC is ERROR. Which is why you don't see this.

My first attempt to improve this tapped into the env var GRPC_GO_LOG_SEVERITY_LEVEL.
However, this had the interesting effect of both forwarding logs to zap and printing gRPC logs in their native log format to stdout/stderr.

Instead, I have now introduced a new flipt compatible log.grpc_level option.
This option defaults to ERROR level. So that the default behaviour is as expected.
However, the grpc log level can always be increased independently up to the main flipt log-level (but not exceed).

see: FLIPT_LOG_GRPC_LEVEL

Normal behaviour is as expected:

Increased gRPC log-level forwards to zap:

Signed-off-by: George MacRorie me@georgemac.com

[codecov-commenter]

Codecov Report

Merging #1029 (686ac8f) into main (4e1cd36) will decrease coverage by 0.12%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##             main    #1029      +/-   ##
==========================================
- Coverage   81.41%   81.28%   -0.13%     
==========================================
  Files          17       17              
  Lines        1813     1817       +4     
==========================================
+ Hits         1476     1477       +1     
- Misses        263      265       +2     
- Partials       74       75       +1     

Impacted Files	Coverage Δ
config/config.go	89.25% <50.00%> (-0.97%)	⬇️

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[markphelps]

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