Re-register metrics only during real update

[khrm]

Before this, metrics were beign registered and unregisterd even when anything in configMap used to change including revision, etc. This causes gap in the metrics and gives us incorrect data over a long time if configmap revision changes.

Assisted-by: Gemini for unit testing

Changes

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Release Notes

NONE

[khrm]

/kind bug

[khrm]

/assign @vdemeester @waveywaves

[khrm]

There was already an old PR from me on this. Instead of necro-bumping that, I recreated a new one.
https://github.com/tektoncd/pipeline/pull/8241/files

There's also a bug related to this. Will search and try to link.

[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage-df to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/pipelinerunmetrics/metrics.go	86.5%	87.4%	0.9
pkg/taskrunmetrics/metrics.go	87.0%	88.1%	1.1

[khrm]

Fixes an old issue here: #7902

[tekton-robot]

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[tekton-robot]

The following is the coverage report on the affected files.
Say /test pull-tekton-pipeline-go-coverage-df to re-run this coverage report

File	Old Coverage	New Coverage	Delta
pkg/pipelinerunmetrics/metrics.go	86.5%	87.4%	0.9
pkg/taskrunmetrics/metrics.go	87.0%	88.1%	1.1

[khrm]

@waveywaves @afrittoli Can we get this merged?

[afrittoli]

Thanks!
/lgtm