Feature Request: Per workload metrics for the TxThrottler

[ejortegau]

Feature Description

This FR proposes building on top of work previously done:

• The transaction throttler was extended in Add basic metrics to vttablet transaction throttler #12418 to include some metrics.
• In Emit per workload labels for existing per table vttablet metrics #12394 we added the ability to collect some metrics on a per-workload name basis.

The TxThrottler metrics are useful, though not very detailed. In particular, they don't allow to know which workload(s) are getting throttled or not. This FR is about extending the TxThrottler metrics to include workload name information.

Use Case(s)

If a keyspace is used by multiple workloads and the TxThrottler is used, this will allow to see how many requests per worklod do indeed get throttled.