services/horizon/ingest: added more description to the history archive metrics labels

[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

added more content to the history archive request metrics

Why

there are sub-labels for the history archive metrics key, and the additional docs help describe what each label is counting.

Known limitations

[jacekn]

Text looks good but I'm not sure how newline will behave in prometheus.
I only saw single line HELP text. It would be good to test this and confirm how output looks like

[Shaptic]

awesome!!

[sreuland]

Text looks good but I'm not sure how newline will behave in prometheus. I only saw single line HELP text. It would be good to test this and confirm how output looks like

yes, the prometheus go lib, coerces all help strings into %q which forces all non-text char's to be printed with escapes, so, I just separated with in-line commas, here's what the final output will look like in metrics output:

# HELP horizon_ingest_history_archive_stats_total Counters of different history archive requests.  'source' label will provide name/address of the physical history archive server from the pool for which a request may be sent.  'type' label will further categorize the potential request into specific requests, 'file_downloads' - the count of files downloaded from an archive server, 'file_uploads' - the count of files uploaded to an archive server, 'requests' - the count of all http requests(includes both queries and file downloads) sent to an archive server, 'cache_hits' - the count of requests for an archive file that were found on local cache instead, no download request sent to archive server.
# TYPE horizon_ingest_history_archive_stats_total counter

[jacekn]

Looks great from my perspective but please get 2nd +1 from somebody with more golang experience