Remove "category" from known events

Problem

When defining a RedisHLL metric there are some keys that need to be defined in the known_events file, category is one of them. To make the creation of metrics easier and have less differences between different types of metric we want to get rid of known_events

Proposal

Evaluate whether:

• category setting provides value, and we need to migrate it so that user's can define it e.g. in the metric's \\\*.yml file
• We have a more up-to-date method to achieve something similar as the provided setting and should migrate users to this
• The setting is only rarely used (or not at all) and should be deprecated completely.

Implementation suggestion can be derived from the outcome of evaluation.

Implementation Suggestion

Anything category was ever used for seems to be gone. As a result, we assume that we are save to remove it.

Testing

Ensure related tests are updated and pipeline is passing