feat(metrics): adding messages queue stats

[geekbrother]

Description

This PR adds metrics for message queue size and statuses:

• Current messages queue size in queued state,
• Current messages queue size in processing state,
• Counter for messages marked as published.

Queue sizes are collected by polling the database with the count query in a minute interval and storing the result in the observable gauge.
The published counter is populated during the message status update.

Following Grafana panels were added:

• Spawned publishing workers count (ref. feat(metrics): adding publishing workers count and errors count #192 ),
• Publishing workers error count (ref feat(metrics): adding publishing workers count and errors count #192 ),
• Count of messages in queued state,
• Count of messages in processing state,
• Published messages counter.

Resolves #189

How Has This Been Tested?

Only local smoke test.

Due Diligence

• Breaking change
• Requires a documentation update
• Requires a e2e/integration test update