Get plugin call stats

Calls, errors and latency of every plugin Contribution invoked for the key's Organization over the window, per plugin version and operation, with the last error code the plugin declared (or plugin_unavailable, invalid_output). Counts are written by each process every few seconds and kept 7 days; a window reads buckets of one resolution (1 minute for 1h, 15 minutes for 24h, 2 hours for 7d) and lists only non-empty ones. Latency percentiles are interpolated from fixed buckets. Requires observability:read on a key that grants every Corpus.