feat: expose cache metrics

[malte-aws]

Description

Adds cached token usage from model providers as new metrics. Reports if tokens are written to cache and if input tokens are read from cache.

My customer asked why we don't expose these metrics.

Documentation PR

strands-agents/docs#125

Type of Change

New feature

Testing

How have you tested the change? Verify that the changes do not break functionality or introduce warnings in consuming repositories: agents-docs, agents-tools, agents-cli

• 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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[Unshure]

Closing as this has already been implemented in a separate PR: #531