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

[ljflores]

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()" in telemetry 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.

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

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[ljflores]

Looks like the assert devices line in send() was added in 964dd93 when typing annotations were introduced, but it was never backported. It's not present in Pacific, so I think this only needs a Quincy backport. I'll still check to see if it reproduces on Pacific though.

[nizamial09]

Thanks @ljflores it works now!

[yaarith]

@ljflores looks good; I had a couple of comments

[yaarith]

adding a note here that this should not be backported since the assert was introduced with the typing changes to the mgr (not available prior to Quincy)

[yaarith]

let's squash the fixes

[ljflores]

Failures, unrelated:
https://tracker.ceph.com/issues/54307
https://tracker.ceph.com/issues/54306
https://tracker.ceph.com/issues/52124

Details:
Bug_#54307: test_cls_rgw.sh: 'index_list_delimited' test times out - Ceph - RGW
Bug_#54306: tasks.cephfs.test_nfs.TestNFS.test_create_multiple_exports: AssertionError: NFS Ganesha cluster deployment failed - Ceph - Orchestrator
Bug_#52124: Invalid read of size 8 in handle_recovery_delete() - Ceph - RADOS