Description

Currently, there's no way to deduce how much of the max capacity is being used in a cache instance. This adds a new Cap() method that enables this. Some cases where this is useful:

• Monitoring the cache usage trend to facilitate better cache sizing
• Decide whether to apply backpressure to data fetching based on the available capacity

Checklist

• Code compiles correctly and linting passes locally
• For all code changes, an entry added to the CHANGELOG.md file describing and linking to
  this PR
• Tests added for new functionality, or regression tests for bug fixes added as applicable