add namespace label/tag to throttling metrics

[gabemontero]

Feature request

Back when implementing #6744 for #6631
we failed to realize that with k8s quota policies being namespace scoped, knowing which namespace the throttled items were
in could have some diagnostic value.

Now that we have been using the metric added for a bit, this realization is now very apparent.

Also, since last touching this space, the original metric was deprecated and
a new one with a shorter name was added. This change only updates the non-deprecated metric with the new label.

Also, I'm uncertain wrt the flexibility we have with adding labels/tags to existing metrics, as in a way it changes behavior. Does the fact that the metric is classified as experimental give us any flexibility? Do we have to go as far as creating a duplicate metric with the new label? I've asked in slack as well at https://tektoncd.slack.com/archives/CLCCEBUMU/p1712853469509439

I will be submitting an initial PR that adds the label to the existing metric that is not deprecated. I am leaving the deprecated metric alone.

Use case

As a tekton administrator, I need to be able to easily see, possibly alert on, whichever subsets of the namespaces in my cluster and experienced k8s throttling of taskruns or pipelineruns with taskruns or custom task runs, etc. etc.

[khrm]

/assign @gabemontero