feat: Instrument message-size for VoteExtensions + ExtendedCommit [BLO-794]

[nivasan1]

In This PR

• I introduce the oracle_message_size metric for VoteExtensions and for ExtendedCommits
• This metric is reported on receipt of a VE in VerifyVoteExtensions and on receipt + verification of a proposed ExtendedCommit in ProcessProposal
• This metric is implemented as a histogram
  • This let's us aggregate all observations (as opposed to a Gauge)
  • This also let's us calculate sample-wide statistics (which we couldn't do w/ a Gauge)
• The buckets chosen are based off experimental observations from our prev. slinky load-tests, they are not final

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (cee70a9) 73.94% compared to head (b7db2c2) 73.98%.

❗ Current head b7db2c2 differs from pull request most recent head 5f9b0b8. Consider uploading reports for the commit 5f9b0b8 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #48      +/-   ##
==========================================
+ Coverage   73.94%   73.98%   +0.04%     
==========================================
  Files         131      125       -6     
  Lines        6766     6547     -219     
==========================================
- Hits         5003     4844     -159     
+ Misses       1434     1390      -44     
+ Partials      329      313      -16     

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