feat: parallelize LoadTest, MonitorTestnet and ObservabilityStack activities

[Zygimantass]

The main idea here is to use signals from LoadTest to tell MonitorTestnet when to stop, so we don't have to depend on arbitrary timeouts to stop the monitoring of a chain.

The only caveat for implementing this technique is that it'd require to split up ObservabilityStack into two activities - one for Prometheus, one for Grafana. This is required since Prometheus is a prerequisite for launching the loadtest (otherwise we're going to have missing metrics for some amount of time).