services/horizon: Strip regexes from routes before sending to prometheus

[paulbellamy]

Fixes #3436

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.
  • The tests are not outstanding, but at least check the regexp. I had a look at testing it at the prometheus level, but didn't see a simple way to do that.
• 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.
  • Didn't see any related docs.

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.
  • Patch change.

What

Remove and escape regexes from chi routes when sending metrics to prometheus.

Why

Prometheus throws and error when metric labels contain unescaped \, ", and \n. See #3436
Also, the regular expressions are ugly and hard to remember when querying.

Known limitations

[TODO or N/A]