[DO NOT MERGE]feat: add instrumentation to forum

[jcapphelix]

Description

This PR adds instrumentation around forum repo.

The need for this comes from us switching from v1 to v2 (Code wise, not backend wise), and the need for that comes from us bringing ulmo.1 changes.

Older implementation of forum cs_comments_service had it's own place in datadog, but that is no longer the case with edx/forum.

Thus this PR works in that direction, we won't have exactly same level of observability as we had cs_comments_service but this makes it more or less similar.

NOTE :- One need not merge this to check on staging, it can stay as is in a branch to check if it works or not in staging, the PR on how to add it in staging is available in "Related PR" section below.

DO NOT MERGE this in release-ulmo without testing and making required changes.

THIS IS POC only.

Useful links

• OLD Forum Service page Datadog
• New Forum dashboard (not in use, not configured)
• Earlier implementation of custom_attr if you go to forum dashboard in it's current state and try to look from March 18 till March 25th, you'll find the references of earlier implementation of custom_attr

Related PR

https://github.com/edx/edx-internal/pull/14147