Add pulsar_tenant and pulsar_namespace labels to prometheus metrics

### Search before asking

- [X] I searched in the [issues](https://github.com/apache/pulsar/issues) and found nothing similar.

### Motivation

It can be useful in to combine metrics by the pulsar tenant or namespace. Currently this is difficult to do by tenant because there is no distinct tenant label. There is only the `namespace` label which has the format `/`.

### Solution

Add two new labels to prometheus metrics: `pulsar_tenant` and `pulsar_namespace`. This would allow metrics to easily be grouped by tenant, and the `pulsar_namespace` label would avoid confusion with the kubernetes namespace. We should also keep the current `namespace` label for compatibility, but consider deprecating it and eventually remove it in some future release.

### Alternatives

_No response_

### Anything else?

_No response_

### Are you willing to submit a PR?

- [X] I'm willing to submit a PR!