op-service: ref-metrics safe for concurrent use

[protolambda]

Description

Fix LatencySeen map updates/reading to be safe for concurrent use.

Bug-report:

t=2025-05-28T04:07:41+0000 lvl=info msg="Emitting local safe update because of L2 block" derivedFrom=0xa8bf89602ff0a73fdf6ca99f7fdb6a709841f328548cd7a70b395d087f48a351:8422261 derived=0xa63f6db0a19a65b5556b16cf27c90c87b3097f54e5c5848c65caad93e91fd252:27477
t=2025-05-28T04:07:41+0000 lvl=info msg="generated attributes in payload queue" txs=1 timestamp=1748405099
fatal error: concurrent map read and map write
goroutine 508 [running]:
github.com/ethereum-optimism/optimism/op-service/metrics.recordRefWithLabels(0xc00064a190, {0x1c1298d, 0xa}, 0x808375, 0x68368bf0, {0xa8, 0xbf, 0x89, 0x60, 0x2f, ...}, ...)
    /app/op-service/metrics/ref_metrics.go:126 +0x154
github.com/ethereum-optimism/optimism/op-service/metrics.(*RefMetrics).RecordRef(...)

[axelKingsley]

I was going to suggest that mu be a RW Lock so that concurrent readers could happen, but seems like the read and write happen at the same time. Maybe that means a CompareAndSwap sort of thing would be possible, but that's just bike shedding.

LGTM.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 7 lines in your changes missing coverage. Please review.

Project coverage is 44.00%. Comparing base (30b7101) to head (108731a).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
op-service/metrics/ref_metrics.go	0.00%	7 Missing ⚠️

❗ There is a different number of reports uploaded between BASE (30b7101) and HEAD (108731a). Click for more details.

HEAD has 4 uploads less than BASE

Flag	BASE (30b7101)	HEAD (108731a)
cannon-go-tests-64	2	0
contracts-bedrock-tests	2	0

Additional details and impacted files

@@             Coverage Diff              @@
##           develop   #16159       +/-   ##
============================================
- Coverage    81.40%   44.00%   -37.40%     
============================================
  Files          161     1209     +1048     
  Lines         8812   101343    +92531     
============================================
+ Hits          7173    44597    +37424     
- Misses        1506    53225    +51719     
- Partials       133     3521     +3388     

Flag	Coverage Δ
cannon-go-tests-64	?
contracts-bedrock-tests	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
op-service/metrics/ref_metrics.go	0.00% <0.00%> (ø)

... and 1369 files with indirect coverage changes

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.