Soong: Fix the package name in metrics.proto file

The metrics.proto needs to be imported to Google3 in order to unmarshal
the build metrics data from bigstore. Cleaned up the enum names to use
the Camel naming convention and renamed the build_metrics to
soong_metrics to be more specific to soong and to allow quering
soong_metrics files from bigstore.

Bug: b/135280521
Test: lunch andf m -j. Checked if soong_metrics was generated correctly
      by using printproto command.

Change-Id: I998c8d05db592e94a653d6ca32250b80df3c9b21