Adds prometheus metrics collection and endpoint

[JPrevost]

Why are these changes being introduced:

• The ability to understand how how timdex api is being used, including which fields are being used and whether any deprecated fields are being requested will help us have necessary information to make decisions going forward
• A culture of assessment is part of our discovery strategic plan. Intentional, specific metric collection is an important part of assessing our platform.

How does this address that need:

• Adds a feature flag, controlled by ENV that enables collection of a few intial metrics and exposes them for collection by prometheus servers

Document any side effects to this change:

• the metrics endpoint is open to anyone wanting to grab metrics

Developer

• All new ENV is documented in README
• All new ENV has been added to Heroku Pipeline, Staging and Prod
• ANDI or Wave has been run in accordance to
  our guide and
  all issues introduced by these changes have been resolved or opened as new
  issues (link to those issues in the Pull Request details above)
• Stakeholder approval has been confirmed (or is not needed)

Code Reviewer

• The commit message is clear and follows our guidelines
  (not just this pull request message)
• There are appropriate tests covering any new functionality
• The documentation has been updated or is unnecessary
• The changes have been verified
• New dependencies are appropriate or there were no changes

Requires database migrations?

NO

Includes new or updated dependencies?

YES