fix task name show anonymous in metric

[chengjoey]

Changes

#8215
fix task name show anonymous in metric

Submitter Checklist

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• Has Docs if any changes are user facing, including updates to minimum requirements e.g. Kubernetes version bumps
• Has Tests included if any functionality added or changed
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Release Notes

fix clusterTask name show `anonymous` in metric

[chengjoey]

/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/taskrunmetrics/metrics.go	86.8%	87.2%	0.4

[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/taskrunmetrics/metrics.go	86.8%	87.2%	0.4

[tekton-robot]

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[chitrangpatel]

/lgtm