gRPC plugin : Rename metrics to stats

[mayurkale22]

At the high level, metrics are used for already aggregated data like Gauge/Counter whereas Stats are for non-aggreagated data (View + Measure + Tags). I think it makes sense to rename metrics to stats for gRPC data recording.
Shortly, we planned to publish specs to explain the diff between these two and when to use Stats/Metrics.

[codecov-io]

Codecov Report

Merging #374 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #374      +/-   ##
==========================================
- Coverage   95.23%   95.21%   -0.02%     
==========================================
  Files         128      128              
  Lines        8492     8492              
  Branches      628      628              
==========================================
- Hits         8087     8086       -1     
- Misses        405      406       +1

Impacted Files	Coverage Δ
src/stackdriver-monitoring.ts	83.15% <0%> (-1.06%)	⬇️
test/test-grpc.ts	99.68% <0%> (ø)	⬆️
src/grpc.ts	93.69% <0%> (ø)	⬆️
src/grpc-stats/server-metrics.ts
src/grpc-stats/client-metrics.ts
src/grpc-stats/server-stats.ts	100% <0%> (ø)
src/grpc-stats/client-stats.ts	100% <0%> (ø)

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