monitoring: remove instance label from ceph-cluster.json completely

monitoring: remove instance label from ceph-cluster.json completely

The `instance` label is only useful if

- the exporter returns only data about its node or instance
- the exporter provides an instance label and then may return data about
  other nodes

In this case, it's about the Prometheus mgr module, which is a single
exporter providing data about a whole cluster, so not only data related
to the node (or instance) the mgr module is running on.  It is
completely irrelevant on which node the exporter runs on, the data
provided doesn't change.  The exporter also doesn't provide `instance`
labels (which Prometheus wouldn't change due to our configuration, see
"honor_labels" setting).

(Actually there's one exception where `instance` labels are provided by
the Ceph mgr module, but that doesn't affect the Ceph Cluster
dashboard.)

Note that keeping that instance label on this particular dashboard would
enable the user to switch between a previously failed mgr instance and
the data collected from there and the currently running mgr instance (on
which the Prometheus mgr module runs on).  That'd split the data, which
I don't think is a useful feature, but rather looks broken.

Fixes: https://tracker.ceph.com/issues/51212
Signed-off-by: Patrick Seidensal <pseidensal@suse.com>