.maintain/monitoring: Add an initial set of Prometheus alerting rules (#6095)

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

substrate/.maintain/monitoring/alerting-rules/alerting-rules.yaml

+groups:
+- name: polkadot.rules
+  rules:
+
+  ##############################################################################
+  # Resource usage
+  ##############################################################################
+
+  - alert: HighCPUUsage
+    expr: polkadot_cpu_usage_percentage >= 100
+    for: 5m
+    labels:
+      severity: warning
+    annotations:
+      message: 'The node {{ $labels.instance }} has a CPU usage higher than 100% for more than 5 minutes'
+
+  ##############################################################################
+  # Block production
+  ##############################################################################
+
+  - alert: LowNumberOfNewBlocks
+    annotations:
+      message: 'Less than one new block per minute on instance {{ $labels.instance }}.'
+    expr: increase(polkadot_block_height{status="best"}[1m]) < 1
+    for: 3m
+    labels:
+      severity: warning
+  - alert: LowNumberOfNewBlocks
+    annotations:
+      message: 'Less than one new block per minute on instance {{ $labels.instance }}.'
+    expr: increase(polkadot_block_height{status="best"}[1m]) < 1
+    for: 10m
+    labels:
+      severity: critical
+
+  ##############################################################################
+  # Block finalization
+  ##############################################################################
+
+  - alert: BlockFinalizationSlow
+    expr: increase(polkadot_block_height{status="finalized"}[1m]) < 1
+    for: 3m
+    labels:
+      severity: warning
+    annotations:
+      message: 'Finalized block on instance {{ $labels.instance }} increases by less than 1 per minute.'
+  - alert: BlockFinalizationSlow
+    expr: increase(polkadot_block_height{status="finalized"}[1m]) < 1
+    for: 10m
+    labels:
+      severity: critical
+    annotations:
+      message: 'Finalized block on instance {{ $labels.instance }} increases by less than 1 per minute.'
+  - alert: BlockFinalizationLaggingBehind
+    # Under the assumption of an average block production of 6 seconds,
+    # "best" and "finalized" being more than 10 blocks apart would imply
+    # more than a 1 minute delay between block production and finalization.
+    expr: (polkadot_block_height_number{status="best"} - ignoring(status) polkadot_block_height_number{status="finalized"}) > 10
+    for: 8m
+    labels:
+      severity: critical
+    annotations:
+      message: "Block finalization on instance {{ $labels.instance }} is behind block production by {{ $value }} for more than 8m"
+
+  ##############################################################################
+  # Transaction queue
+  ##############################################################################
+
+  - alert: TransactionQueueSize
+    expr: polkadot_sub_txpool_validations_scheduled - polkadot_sub_txpool_validations_finished > 10
+    for: 10m
+    labels:
+      severity: warning
+    annotations:
+      message: 'The node {{ $labels.instance }} has more than 10 transactions in the queue for more than 10 minutes'
+  - alert: TransactionQueueSize
+    expr: polkadot_sub_txpool_validations_scheduled - polkadot_sub_txpool_validations_finished > 10
+    for: 30m
+    labels:
+      severity: critical
+    annotations:
+      message: 'The node {{ $labels.instance }} has more than 10 transactions in the queue for more than 30 minutes'
+
+  ##############################################################################
+  # Networking
+  ##############################################################################
+
+  - alert: LowNumberOfPeers
+    expr: polkadot_sub_libp2p_peers_count < 3
+    for: 3m
+    labels:
+      severity: warning
+    annotations:
+      message: 'The node {{ $labels.instance }} has less than 3 peers for more than 3 minutes'
+  - alert: LowNumberOfPeers
+    expr: polkadot_sub_libp2p_peers_count < 3
+    for: 15m
+    labels:
+      severity: critical
+    annotations:
+      message: 'The node {{ $labels.instance }} has less than 3 peers for more than 15 minutes'
+
+  ##############################################################################
+  # Others
+  ##############################################################################
+
+  - alert: AuthorityDiscoveryHighDiscoveryFailure
+    expr: polkadot_authority_discovery_handle_value_found_event_failure / ignoring(name) polkadot_authority_discovery_dht_event_received{name="value_found"} > 0.5
+    for: 2h
+    labels:
+      severity: warning
+    annotations:
+      message: "Authority discovery on node {{ $labels.instance }} fails to process more than 50 % of the values found on the DHT."