Introduce ff for deprecating Monitor Metrics in sidebar and settings

What does this MR do and why?

Describe in detail what your merge request does and why.

This is the first part of hiding Monitor Metrics related views. The aim is deprecate the feature with the use of a feature flag and than proceed with the actual removal. This MR includes:

• hide Monitor > Metrics view and left navigation item
• hide Settings > Monitor > Metrics
• hide Settings > Monitor > Grafana authentication
• hide Settings > Integrations > Prometheus > Metrics (project level)
• hide Settings > General > Visibility, project features, permissions > Monitor > Metrics Dashboard

Screenshots or screen recordings

Screenshots are required for UI changes, and strongly recommended for all other merge requests.

feature flag disabled	feature flag enabled

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

This part doesn't require any specific setup. However, you will be able to only see the empty states if you haven't used Metrics on your local environment yet.

Check these views:

• Monitor > Metrics view and left navigation item (Clicking top level navbar item, Monitor, should redirect to the first subitem: Metrics when the flag is disabled, Error tracking when the flag is enabled)
• Settings > Monitor > Metrics
• Settings > Monitor > Grafana authentication
• Settings > Integrations > Prometheus > Metrics (project level)
• Settings > General > Visibility, project features, permissions > Monitor > Metrics Dashboard

To enable the flag type this into terminal:

rails c
Feature.enable(:remove_monitor_metrics) 

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #399231 (closed)