SS-1322 monitor connection pool

[hamzaimran08]

Description

This PR relates to SS-1322 and SS-1929.

Adds connection pool observability for Serve so we can inspect Django-side PostgreSQL pool during outages without exec-ing into pods and also request behavior.

• Adds a /metrics endpoint gated by PROMETHEUS_METRICS_ENABLED; when disabled, the endpoint returns 404.
• Exposes Django/psycopg pool values as Prometheus gauges, labelled by database alias, pod name, process id, and pool id.
• Tracks pool enabled/opened state, configured min/max size, current size, available connections, waiting requests, served requests, and pool request errors.
• Adds Django HTTP request metrics:
  • django_http_requests_total
  • django_http_request_duration_seconds
  • django_http_request_exceptions_total
• Records HTTP metrics with labels: method, resolved route, status code, exception type, and pod.
• Adds optional DB pool stats logging through DB_POOL_STATS_LOGGING_ENABLED, logging only when relevant pool values change.
• Includes matching serve-charts configuration so Helm can enable the metrics endpoint and Prometheus scraping while keeping /metrics blocked from public ingress.

Checklist

• This pull request is against develop branch (not applicable for hotfixes)
• I have included a link to the issue on GitHub or JIRA (if any)
• I have included migration files (if there are changes to the model classes)
• I have added or updated unit and end2end tests or a manual test case to complement my changes
• I have ran unit and end2end tests
• I have updated the related documentation (if necessary)
• I have added a reviewer for this pull request
• I have added myself as an author for this pull request
• In the case I have modified settings.py, then I have also updated the studio-settings-configmap.yaml file in serve-charts
• In case your changes are large enough, did you deploy your changes to develop instance?