OpenTelemetry metrics: Add a metric counter for total deduplicated messages

[mauroservienti]

Describe the suggested improvement

Is your improvement related to a problem? Please describe.

Currently, NServiceBus reports the following metric counters:

• Total fetched messages
• Total number of messages successfully processed
• Total number of messages where processing failed

As of today, there is a mismatch between the number of fetched messages and the sum of successes and failures because we do not count deduplicated messages.

Describe the suggested solution

Math says that number of fetched messages should be the sum of "failure + success + deduplicated messages". Hence, exposing a counter for the number of deduplicated messages would be convenient. Note that we already add to the OpenTelemetry activity a tag signaling the message is a duplicate.

Additional Context

No response