Indexing and serving metrics are exported via Prometheus:
$ sudo apt install prometheus
$ echo "
scrape_configs:
- job_name: electrs
static_configs:
- targets: ['localhost:4224']
" | sudo tee -a /etc/prometheus/prometheus.yml
$ sudo systemctl restart prometheus
$ firefox 'http://localhost:9090/graph?g0.range_input=1h&g0.expr=index_height&g0.tab=0'