services/horizon: Add latest ledger closed_at metrics

[bartekn]

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

This commit adds:

• history_latest_ledger_closed_at to root resource that determines the latest ingested ledger close time,
• horizon_history_latest_ledger_closed_ago_seconds to /metrics that return time in seconds since the latest ingested ledger close time.

Close #3206.

Why

core_latest_ledger - history_latest_ledger doesn't always determine if Horizon is fully sync with the network. It's possible that underlying Core instance is behind or stuck.