http archive requests include user agent and metrics

[sreuland]

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

fixed the history archive config for http backend to include custom user-agent value.
added history_archive_stats_total metrics for counters on requests and downloads of files from Archive, most of changes in the pr are related to enabling the metrics.

Why

visibility on how much the archives are being used by horizon ingestion.
Closes #5161

Known limitations

this is targeting the 2.28.0 release branch, it will be ported to master as part of porting the final release back to master later.