* Add Prometheus alerts if unbounded channels are too large

* Tweaks

* .maintain/monitoring: Add alert when continuous task ends

Through the `polkadot_tasks_ended_total` Prometheus metric one can tell
when a task ended. Use this metric to alert when specific
known-to-be-continuous tasks end on a node.

* .maintain/monitoring: Don't hard-code task names

* .maintain/monitoring: Normalize alerting rules

- Start alert names with their component and end with the describing
adjective.

- Describe alert duration in `message` with `for more than` across all
alerts.

* .maintain/monitoring: Fix alert tests

Alert on high file descriptor allocation.

The `HighCPUUsage` alert is based on the `cpu_usage_percentage` metric.
Instead of exposing the overall CPU usage in percent, the metric exposes
the per core usage summed over all cores.

This commit removes the alert for two reasons:

1. Substrate itself does not expose the core count and thus one can not
alert based on the `cpu_usage_percentage` metric.

2. Alerting based on CPU usage is generic and not specific to Substrate
or Blockchains. Thus any CPU usage alert suffice.

The transaction queue size alert has been firing with a constant 10
transactions in the queue. While maybe problematic those 10 transactions
don't need to be the same across scrape intervals.

Instead of alerting with a size above 10, alert based on two things:

1. Monotonically increasing queue size

2. Upper limit queue size reached

* .maintain/monitoring: Add alerting rule tests

* .maintain/monitoring/alerting-rules/alerting-rules.yaml: Break lines

* .gitlab-ci.yml: Add promtool rule testing step

Create a place to collaborate on Prometheus alerting rules for
Substrate starting with a basic set of rules covering:

- Resource usage
- Block production
- Block finalization
- Transaction queue
- Networking
- ... Others