Enable bazel build for metrics proto files

[lalitb]

Changes

~/bazel/opentelemetry_proto.BUILD file only contains the build rules for traces and logs. Adding build rules for metrics too.

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Unit tests have been added
• Changes in public API reviewed

[ThomsonTan]

Is OTLP gRPC metrics exporter enabled in bazel?

[codecov]

Codecov Report

Merging #1489 (936b8c7) into main (235c90e) will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##             main    #1489      +/-   ##
==========================================
+ Coverage   84.65%   84.67%   +0.03%     
==========================================
  Files         155      155              
  Lines        4787     4787              
==========================================
+ Hits         4052     4053       +1     
+ Misses        735      734       -1     

Impacted Files	Coverage Δ
sdk/src/trace/batch_span_processor.cc	91.41% <0.00%> (+0.79%)	⬆️

[lalitb]

Is OTLP gRPC metrics exporter enabled in bazel?

Not yet, will create a PR for that once this is merged.