Improve Prometheus and Openmetrics codebase

[ChrsMark]

After adding textparse library with #33865 and we have verified it doesn't break any functionality we can further improve our codebase and organize the helper libs in a better way. Some suggestions:

1. Use interface at: Use textparse in prometheus module helper library #33865 (comment)
2. Split codeflow in smaller functions: Use textparse in prometheus module helper library #33865 (comment)
3. Look for duplicated implementations between Prometheus and Openmetrics helper libs and consider abstracting them into a common helper lib.

[botelastic]

Hi!
We just realized that we haven't looked into this issue in a while. We're sorry!

We're labeling this issue as Stale to make it hit our filters and make sure we get back to it as soon as possible. In the meantime, it'd be extremely helpful if you could take a look at it as well and confirm its relevance. A simple comment with a nice emoji will be enough :+1.
Thank you for your contribution!