Encapsulate supplier, monitor and metric naming logic in some common TestMetric type

After an offline discussion together with @mwalenia we decided to create concrete classes for each metric type (Item_count, byte_count, time). Each class like this will contain:
- metric name
- supplier for the metric
- monitor for the metric

It turns out that all this (along with the monitor/supplier can be encapsulated and then attached to the pipeline/metrics reading where needed. This will also encapsulate the naming logic (so that there are no typos again).

Imported from Jira [BEAM-7245](https://issues.apache.org/jira/browse/BEAM-7245). Original Jira may contain additional context.
Reported by: ŁukaszG.

Start by reviewing the existing metric, supplier, and monitor naming logic used where pipeline metrics are read; the issue does not name specific files or tests. Use the original BEAM-7245 discussion for context, then define concrete Item_count, byte_count, and time metric types that encapsulate the metric name, supplier, and monitor, with naming handled consistently.

Written by the indexing model from the issue text.