Improve latency metrics

Right now all we display is upload latency. We could display additional latency metrics from GraceDB to get a fuller look at the downstream alert infrastructure latency:

• latency of PASTRO_READY label
• latency of SKYMAP_READY label
• latency of PE reports, DQ reports, etc.

Everything uploaded to GraceDB should be timestamped so in principle we could collect these metrics for any metadata applied.