services/horizon/internal/ingest: Add metrics for ingestion loaders

[tamirms]

PR Checklist

PR Structure

• This PR has reasonably narrow scope (if not, break it down into smaller PRs).
• This PR avoids mixing refactoring changes with feature changes (split into two PRs
  otherwise).
• This PR's title starts with name of package that is most changed in the PR, ex.
  services/friendbot, or all or doc if the changes are broad or impact many
  packages.

Thoroughness

• This PR adds tests for the most critical parts of the new functionality or fixes.
• I've updated any docs (developer docs, .md
  files, etc... affected by this change). Take a look in the docs folder for a given service,
  like this one.

Release planning

• I've updated the relevant CHANGELOG (here for Horizon) if
  needed with deprecations, added features, breaking changes, and DB schema changes.
• I've decided if this PR requires a new major/minor version according to
  semver, or if it's mainly a patch change. The PR is targeted at the next
  release branch if it's not a patch change.

What

Close #5154

We do not have any metrics capturing how long it takes to run the ingestion loaders:

https://github.com/stellar/go/blob/release-horizon-v2.28.0/services/horizon/internal/ingest/group_processors.go#L101-L105

This commit adds prometheus metrics for the duration of each loader so we can determine how much of ingestion is spent in the loaders vs the processors. We also added prometheus metrics capturing how many records were queried and inserted during the execution of each loader.

Known limitations

[N/A]

[sreuland]

since 2.28.3 went out last week unfortunately on a bit of rush, this needs to target master now

[sreuland]

lgtm!

[tamirms]

@sreuland I think I have addressed all your feedback, could you take another look?

[sreuland]

@sreuland I think I have addressed all your feedback, could you take another look?

looks good, and thanks for consideration on suggestions!

do need to re-target this pr to merge to master the 2.28.3 branch has already been released.