Darren/chore/backport metrics

[darrenvechain]

Description

Backport maindb v4 metrics

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes
• New and existing E2E tests pass locally with my changes
• Any dependent changes have been merged and published in downstream modules
• I have not added any vulnerable dependencies to my code

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 65.75342% with 25 lines in your changes missing coverage. Please review.

Project coverage is 60.66%. Comparing base (3aa2935) to head (00422d7).

Files with missing lines	Patch %	Lines
metrics/prometheus.go	68.88%	10 Missing and 4 partials ⚠️
muxdb/internal/trie/cache.go	0.00%	11 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #909      +/-   ##
==========================================
+ Coverage   60.63%   60.66%   +0.03%     
==========================================
  Files         215      215              
  Lines       23141    23200      +59     
==========================================
+ Hits        14031    14075      +44     
- Misses       7960     7971      +11     
- Partials     1150     1154       +4     

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[otherview]

Given the context is available we can probably do something like

Suggested change

	var blk interface{}
	var cached bool
	if blk, cached, err = r.caches.summaries.GetOrLoad(id, func() (interface{}, error) {
	var blk interface{}
	cached := true
	if blk, err = r.caches.summaries.GetOrLoad(id, func() (interface{}, error) {
	cached = false