Add inmemory cache metrics

[khanhtc1202]

What this PR does / why we need it:

Which issue(s) this PR fixes:

Fixes #

Does this PR introduce a user-facing change?:

NONE

[nakabonne]

👍
/lgtm

[pipecd-bot]

Code coverage for golang is 32.60%. This pull request increases coverage by 0.02%.

File	Function	Base	Head	Diff
pkg/cache/cachemetrics/metrics.go	IncGetOperationCounter	0.00%	100.00%	+100.00%
pkg/cache/memorycache/cache.go	Cache.Get	0.00%	0.00%	+0.00%
pkg/cache/memorycache/lru_cache.go	LRUCache.Get	75.00%	66.67%	-8.33%
pkg/cache/memorycache/ttl_cache.go	TTLCache.Get	100.00%	100.00%	+0.00%

[nghialv]

/approve

[pipecd-bot]

This pull request is APPROVED by nghialv.

Approvers can cancel the approval by writing /approve cancel in a comment. Any additional commits also will change this pull request to be not-approved.