feat: Add Alloy metrics scraping and related configurations

General MR

Summary

As a user I would like to use alloy-metrics collector to scrape ServiceMonitor metrics instead of prometheus.

In order to start the transition to use alloy-metrics as the default metrics scraper instead of prometheus we will need to:

1. If a user enables the monitoring package and alloy-metrics with scrapeConfig enabled, alloy will collect the metrics and publish to prometheus, In the same config BB should disable prometheus from scraping metrics to not collect metrics twice.
2. If a user enables monitoring and mimir with alloy-metrics scrapeConfig enabled, then alloy should push the metrics to both prometheus and mimir. (Do not configure prometheus to push to mimir)
3. Have the ability to have alloy-metrics just publish metrics to mimir and not prometheus.

Relevant logs/screenshots

Linked Issue

issue

Upgrade Notices

Alloy Metrics Scraping: Big Bang now supports using alloy-metrics as an alternative metrics scraper to Prometheus. Alloy autodiscovers ServiceMonitors/PodMonitors and forwards metrics via remote write to Prometheus and/or Mimir.

To enable alloy-metrics and disable Prometheus scraping (recommended to avoid duplicate metrics):

monitoring:
  enabled: true
  prometheusMetrics:
    enabled: false

alloy:
  enabled: true
  alloyMetrics:
    enabled: true

NOTE: When prometheusMetrics.enabled=false, the Prometheus Targets UI will show no active scrape targets. PromQL queries still work as Prometheus receives metrics from Alloy via remote write.

WARNING: Enabling both alloy.alloyMetrics.enabled=true and monitoring.prometheusMetrics.enabled=true simultaneously will result in duplicate metrics collection.