feat: add a new metric for tracking count of successfully merged messages

[aditiharini]

Why is this change needed?

We're currently using hubble.hub.merge_message.count for the message count metric and it looks skewed because some hubs have more messages submitted as bundles than others and each bundle only count as a single message rather than as the number of individual messages inside them. Create a new metric to track successfully merged messages (we already have a metric for errors).

Merge Checklist

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PR-Codex overview

The focus of this PR is to increment a statsd metric when a message submission is successful in the Hub class of the Hubble app.

Detailed summary

• Increment statsd metric on successful message submission in submit_message method.
• Added statsd metric incrementation in the mergeResult match block.

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⚠️ No Changeset found

Latest commit: b2ef13e

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Name	Status	Preview	Comments	Updated (UTC)
hub-monorepo	✅ Ready (Inspect)	Visit Preview	💬 Add feedback	Sep 3, 2024 10:04pm

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 44.34%. Comparing base (1880070) to head (b2ef13e).
Report is 336 commits behind head on main.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #2290       +/-   ##
===========================================
- Coverage   74.05%   44.34%   -29.72%     
===========================================
  Files          99      146       +47     
  Lines        9425    25474    +16049     
  Branches     2097     9059     +6962     
===========================================
+ Hits         6980    11297     +4317     
- Misses       2327    12427    +10100     
- Partials      118     1750     +1632     

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