alert metrics: add chart to display time-to-acknowledge and time-to-close #2313
Description
What problem would you like to solve? Please describe:
Visualize time-to-ack and time-to-close over time
Describe the solution you'd like:
- A new line chart with 1 line for average TTC, 1 for average TTA
- Where TTA is NULL, use TTC (TTA can really be understood as time-to-not-be-unacknowledged)