pybind/mgr/prometheus: Add instance_id metadata for rgw

pybind/mgr/prometheus: Add instance_id metadata for rgw

In order to get the `ceph_daemon` label for `rgw` metrics corresponding to the
value before #40220, we need to add the `instance_id` label to the
`ceph_rgw_metadata` metric.

This way, the old `ceph_daemon` label can be added to any `ceph_rgw_*` metric
using the following PromQL query, for instance:

```
ceph_rgw_req * on (instance_id) group_left(ceph_daemon) ceph_rgw_metadata
```

Signed-off-by: Benoît Knecht <bknecht@protonmail.ch>