Add `taskType`/`category` to task related metrics

### Description

I would like to see the `taskType` label/dimension on the task related metrics e.g. `task/pending/count`.
To have the `category` label/dimension on the task related metrics would also help, to know on which middlemanager category a task is being placed.

### Motivation

I would like to be able to scale my middlemanagers via an HPA.
My compaction middlemanagers are separated and thus I'd need to exclude pending task count from my scaling query when the taskType is `compact`/`partial_dimension_distribution`/`partial_index_generic_merge`/`partial_range_index_generate`.

Start by locating the task-related metrics, including task/pending/count, and trace how their labels or dimensions are defined. Done means the requested taskType and category dimensions are available on task-related metrics, including the values needed to distinguish compaction tasks.

Written by the indexing model from the issue text.