Added metrics for LRU data cache

[AshishNaware]

Fixes #679

Description

This PR adds additional metrics around LRU data cache such as size, capacity, evictions and cache miss in order to have a same set of metrics collection across the board.

Changelog

Add metrics for LRU data cache

[lambdanis]

Thank you @AshishNaware, overall looks good to me. I left two comments about small details.

Tetragon metrics are also documented in a generated reference doc, so the new metrics must be added there too. To generate the metrics reference, run:

make metrics-docs

Then you should see updated doc files.

As a side note, at some point it might be nice to refactor, to have both process and observer cache use the same cache implementation, but with separate metrics. But this is out of scope in this PR.

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[lambdanis]

looks good to me, thanks!

[lambdanis]

@AshishNaware Could you please squash the three commits and force push? That's a practice in the Tetragon project, to keep clean git history on the main branch. Currently one of the CI jobs is failing because one of the commit messages is too long.

[AshishNaware]

@lambdanis @mtardy - I have squashed the commits and rebased the branch. This PR has now passed all the checks.