[alertmanager] Add ServiceMonitor support to Prometheus Alertmanager Helm Chart

[sajad-sadra]

Is your feature request related to a problem ?

Yes, the feature request is related to the challenge of integrating Alertmanager metrics scraping with Prometheus when using Prometheus-Operator. Currently, users must manually create a ServiceMonitor resource to scrape metrics from the Alertmanager instance. This extra step adds complexity and can be error-prone, especially for users unfamiliar with ServiceMonitor configurations.

Describe the solution you'd like.

I propose adding a ServiceMonitor resource to the Prometheus Alertmanager Helm chart. This resource would be conditionally created based on a configurable value (serviceMonitor.enabled). Users should also be able to customize its behavior through additional Helm chart values such as serviceMonitor.interval.

Describe alternatives you've considered.

2. Manually Creating ServiceMonitor:
   Users can create their own ServiceMonitor resource outside of the Helm chart. However, this adds complexity and goes against the goal of providing an all-in-one deployment solution.

3. Using Annotations for Metrics Scraping:
   Some Prometheus setups scrape metrics directly from service endpoints using annotations. While this works, it lacks the advanced features and flexibility provided by ServiceMonitor, such as relabeling and namespace isolation.

Additional context.

No response