Bucket entry counters and metrics

[ThomasBrady]

Description

Resolves #4426

• Adds BucketEntryCounters to BucketIndexImpl to count entry types and duration.
• Adds functionality to BucketList to accumulate the counters for all buckets
• Adds functionality to LedgerManager to emit these metrics to medida

Still TODO:

• Local testing of serialization of indexes with counters

Out of scope:

• XDR Changes for including counters in ledger close meta
• grafana updates

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[SirTyson]

Looks great! A lot of super small nit picky things. I'll go ahead and approve now so you can merge after a few fixups next week when I'm out.

[SirTyson]

A few more changes. Also, these new metrics should be added to metrics.md.

[SirTyson]

LGTM! A couple last super nit picky things then we can merge.

[ThomasBrady]

This PR is the dashboard changes FYI https://github.com/stellar/puppet-v4/pull/4081

[SirTyson]

Last round of changes looks good! Once we have a serialization test, we'll be ready to merge.

[ThomasBrady]

Last round of changes looks good! Once we have a serialization test, we'll be ready to merge.

I'm pretty sure this is already tested by "serialize bucket indexes", but I added some extra assertions to make sure the counters are non-empty before the equality comparison.