Built-in ServiceMonitor for PgBouncer and StatsD

### Official Helm Chart version 1.7.0 (latest released) ### Apache Airflow version 2.3.0 ### Kubernetes Version v1.24.x ### Helm Chart configuration _No response_ ### Docker Image customisations _No response_ ### What happened The current version of the chart as is doesn't have support for built-in `ServiceMonitor`. Is this a deliberate omission because it's considered bloat(as it isn't really required by Airflow, and also as that would mean tighter integration with Prometheus- and we don't want to outright do that for our users?). Or can this be added as a feature/enhancement to the upstream chart? Since, Prometheus is a major CNCF project which is one of the most widely used solution for monitoring. I am willing to submit a PR, if this seems like a valid issue :) ### What you think should happen instead _No response_ ### How to reproduce _No response_ ### Anything else _No response_ ### Are you willing to submit PR? - [X] Yes I am willing to submit a PR! ### Code of Conduct - [X] I agree to follow this project's [Code of Conduct](https://github.com/apache/airflow/blob/main/CODE_OF_CONDUCT.md)