[improve][broker] PIP-406: Introduce pulsar_subscription_dispatch_throttled_msgs and bytes metrics

[shibd]

Motivation

Refer to #23945

Modifications

1. Maintain dispatchThrottledMsgs and dispatchThrottledBytes in AbstractBaseDispatcher. Increase these values whenever the number of messages/bytes is reduced during calculateToRead.
2. Output these fields when retrieving topic stats and metrics.

Verifying this change

• Add testSubscriptionStatsDispatchThrottled to cover it

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete