mgr/telemetry: handle empty device report when "send" is triggered

mgr/telemetry: handle empty device report when "send" is triggered

On certain environments, such as the "ceph-dev-docker" environment
(https://github.com/ricardoasmarques/ceph-dev-docker), the mgr
module is unable to fetch device metrics. As a result, the device
report generated by "gather_device_report()" returns an empty dict.
This causes an AssertionError when the "send" function is triggered
(i.e. by running `ceph telemetry status` or `ceph telemetry send`),
and the module crashes.

The fix in this commit checks that the generated device report
contains metrics before trying to send it. If the device report
does not contain metrics (it returns an empty dict), the module
will log an appropriate message in the mgr log and not send the
device report.

If this scenario happens when running the `ceph telemetry send` command,
the user will additionally see this message:
```
Ceph report sent to https://telemetry.ceph.com/report
Unable to send device report: channel is on, but generated report was empty.
```

I also added a few more debug messages in gather_device_report() to make
future debugging easier.

Fixes: https://tracker.ceph.com/issues/54250
Signed-off-by: Laura Flores <lflores@redhat.com>