feat(issue-12348): add validator index label to validator_statuses me…

[0xste]

What type of PR is this?
Feature

What does this PR do? Why is it needed?

• We don't currently have a way to retrieve validator index from prometheus
• It's useful for joining to various other metrics and, an example use-case is linking from a dashboard using the index as part of the target link i.e. https://beaconcha.in/validator/1101253 or joining with another metric to get additional metadata
• This has been added to the existing validator_statuses metric:
  • We don't have a redundant info metric & associated cardinality
  • We make a non-breaking change to the metric contract for the existing validator_statuses metric by adding an additional label

Which issues(s) does this PR fix?
Fixes #12348

Acknowledgements

• I have read CONTRIBUTING.md.
• I have made an appropriate entry to CHANGELOG.md.
• I have added a description to this PR with sufficient context for reviewers to understand this PR.

[james-prysm]

looks like there is this panic error in e2e runs

panic: inconsistent label cardinality: expected 2 label values but got 1 in []string{"0xb7eaf282595bd590bde41f67783d12ccf7666aea2f1efbaeaa80c8478a157cf59ca7bf009e5a125163212b0b9f51c876"}

goroutine 307 [running]:
github.com/prometheus/client_golang/prometheus.(*GaugeVec).WithLabelValues(...)
	external/com_github_prometheus_client_golang/prometheus/gauge.go:238
github.com/prysmaticlabs/prysm/v5/validator/client.(*validator).logDuties(0xc00067c488, 0x0, {0xc000367b08, 0x80, 0x0?}, {0xc000499b08, 0x80, 0x0?})
	validator/client/validator.go:973 +0x17d6
github.com/prysmaticlabs/prysm/v5/validator/client.(*validator).UpdateDuties(0xc00067c488, {0x1aea038, 0xc000aec120}, 0x0)
	validator/client/validator.go:567 +0x75e
github.com/prysmaticlabs/prysm/v5/validator/client.run({0x1aea038, 0xc000aec120}, {0x1b0f4f0, 0xc00067c488})
	validator/client/runner.go:46 +0xe2
created by github.com/prysmaticlabs/prysm/v5/validator/client.(*ValidatorService).Start in goroutine 293
	validator/client/service.go:221 +0xd0b
	```

panic in e2e

[0xste]

@james-prysm had missed the second invocation of WithLabelValues

I didn't find how to run e2e tests locally in the contributing guide

[james-prysm]

@0xste you can run e2e with bazel bazel test //testing/endtoend:go_default_test --//proto:network=minimal --test_filter=TestEndToEnd_MinimalConfig --test_timeout=10000 --test_output=streamed --flaky_test_attempts=1

we should update that, will note