Emit message handler time metric

[saratry]

• depends on https://github.com/Particular/NServiceBus/pull/7077

This PR introduces a new metric for measuring the message handler time using an OpenTelemetry-native metric. We introduced a new tag/label to record the message handler type (nservicebus.message_handler_type).

• [x] Update public docs to include handler time.
• [x] Document Message handler time, which purely measures the time it takes to execute the handler code
• [x] Adjust the Processing time in the Metrics package documentation to include serialization, saga loading time, etc.

[danielmarbach]

I made a larger comment on the other PR I was pinged on that is based on the work here https://github.com/Particular/NServiceBus/pull/7064#issuecomment-2173060785

[mauroservienti]

Reverting to draft to signal the dependency on https://github.com/Particular/NServiceBus/pull/7077 and preventing accidental merge

[mauroservienti]

@lailabougria @SzymonPobiega, this is ready for a review

[SzymonPobiega]

Replaced by https://github.com/Particular/NServiceBus/pull/7095