Additional metrics exported from Celery workers

[rbagd]

Description

This PR includes an enhancement regarding metrics exported from Celery workers and implements these measurements

• Prefetched task up-down counter (number of tasks queued locally in the worker) and time spent in prefetch mode
• Active task up-down counter and task processing duration

Currently, there's only one metric being exported - flower.task.runtime.seconds, and it was renamed to follow semantic conventions (reported in the changelog).

Changes in this PR also fix a memory leak present in current instrumentation.

Fixes #3458

Type of change

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

How Has This Been Tested?

• Unit tests added for all newly included and updated metrics

Does This PR Require a Core Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added