Promtimer fails if logs have no persistent blocks

[jameseh96]

A node which has not been up for long may not have any persistent blocks.

That is, stats_snapshot may just have head chunks and the WAL:

$ tree ./stats_snapshot/
./stats_snapshot/
├── chunks_head
│   └── 000001
├── queries.active
└── wal
    └── 00000000

In this case, Promtimer fails with:

  File ".../promtimer/promtimer.py", line 308, in <module>
    main()
  File ".../promtimer/promtimer.py", line 259, in main
    times = cbstats.CBCollect.compute_min_and_max_times(stats_sources)
  File ".../promtimer/promtimer/cbstats.py", line 345, in compute_min_and_max_times
    times = [s.get_min_and_max_times() for s in sources]
  File ".../promtimer/promtimer/cbstats.py", line 345, in <listcomp>
    times = [s.get_min_and_max_times() for s in sources]
  File ".../promtimer/promtimer/cbstats.py", line 175, in get_min_and_max_times
    return get_prometheus_times(self._cbcollect_dir)
  File ".../promtimer/promtimer/cbstats.py", line 570, in get_prometheus_times
    return min(min_times), max(max_times)
ValueError: min() arg is an empty sequence

Corresponding to:

def get_prometheus_times(cbcollect_dir):
    min_times = []
    max_times = []
    meta_files = glob.glob(path.join(cbcollect_dir, 'stats_snapshot', '*', 'meta.json'))
    for meta_file in meta_files:
        with open(meta_file, 'r') as file:
            meta = json.loads(file.read())
            min_times.append(meta['minTime'] / 1000.0)
            max_times.append(meta['maxTime'] / 1000.0)
    return min(min_times), max(max_times)

No persistent block dirs like 01F5WQSJ4705B6SRTQE4CCB0DZ (containing meta.json) are present, so the glob does not match anything.