[WIP] Re-register metrics only during real updation

[khrm]

Before this, metrics were beign registered and unregisterd even when anything in configMap used to change including revision, etc.

Changes

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[khrm]

/kind bug

[tekton-robot]

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

File	Old Coverage	New Coverage	Delta
pkg/pipelinerunmetrics/metrics.go	85.4%	85.0%	-0.4
pkg/taskrunmetrics/metrics.go	87.2%	86.9%	-0.3

[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	85.4%	85.0%	-0.4
pkg/taskrunmetrics/metrics.go	87.2%	86.9%	-0.3

[tekton-robot]

@khrm: The following tests failed, say /retest to rerun all failed tests or /retest-required to rerun all mandatory failed tests:

Test name	Commit	Details	Required	Rerun command
pull-tekton-pipeline-beta-integration-tests	2916734	link	true	/test pull-tekton-pipeline-beta-integration-tests
pull-tekton-pipeline-alpha-integration-tests	2916734	link	true	/test pull-tekton-pipeline-alpha-integration-tests

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