Attach usage and metrics to MessageAddedEvent.

[esoler-sage]

Description

This PR attaches usage and metrics to MessageAddedEvent.

Hooks are a very powerful way to work with events during the event loop. One of those is the message added event.

With this change, it enables a some use cases, like storing values in a database, understand session token usage / conversation cost, etc.

Related Issues

#1532
#1503
#1197

Type of Change

New feature

Testing

• I ran hatch run prepare

Checklist

• I have read the CONTRIBUTING document
• I have added any necessary tests that prove my fix is effective or my feature works
• I have updated the documentation accordingly
• I have added an appropriate example to the documentation to outline the feature, or no new docs are needed
• My changes generate no new warnings
• Any dependent changes have been merged and published

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