mgr/telemetry: down osds result in empty perf report fields
If there is a problem extracting data from a particular osd (i.e. an osd is down), some fields in
the telemetry report show up empty. This is because some functions exit early and return empty JSON objects
upon encountering a down osd rather than continuing and collecting whatever data is available.
The fields that are affected by this bug are "osd_perf_histograms", "mempool", and "heap stats".