feat: add postgres/gitea/blackbox alerts and more node alerts

environments/dev/adibrov/alerts/postgres/postgres_down.yaml

@@ -0,0 +1,19 @@
+name: "DEV ADIBROV - PostgreSQL Down"
+expression: |
+  pg_up{job="postgres"}
+threshold: 1
+for: "2m"
+condition_type: "lt"
+need_reduce: true
+reducer_type: "min"
+no_data_state: "Alerting"
+exec_err_state: "Alerting"
+labels:
+  service: "postgres"
+  severity: "critical"
+  team: "infra"
+summary: |
+  PostgreSQL недоступен на {{ $labels.instance }}
+description: |
+  Exporter не может подключиться к PostgreSQL на {{ $labels.instance }}.
+  База данных либо упала, либо недоступна по сети.

environments/dev/adibrov/alerts/postgres/postgres_long_transactions.yaml

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+name: "DEV ADIBROV - PostgreSQL Long Running Transactions"
+expression: |
+  max by(instance) (
+    pg_stat_activity_max_tx_duration{job="postgres", state="active"}
+  )
+threshold: 300
+for: "5m"
+condition_type: "gt"
+need_reduce: true
+reducer_type: "max"
+no_data_state: "OK"
+exec_err_state: "Error"
+labels:
+  service: "postgres"
+  severity: "warning"
+  team: "infra"
+summary: |
+  Долгая транзакция в PostgreSQL на {{ $labels.instance }}: {{ printf "%.0f" $values.B.Value }}с
+description: |
+  На {{ $labels.instance }} есть транзакция, выполняющаяся более 5 минут.
+  Долгие транзакции блокируют vacuum и могут накапливать bloat.

environments/dev/adibrov/alerts/postgres/postgres_too_many_connections.yaml

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+name: "DEV ADIBROV - PostgreSQL Too Many Connections"
+expression: |
+  (
+    pg_stat_activity_count{job="postgres"}
+    / pg_settings_max_connections{job="postgres"}
+  ) * 100
+threshold: 80
+for: "5m"
+condition_type: "gt"
+need_reduce: true
+reducer_type: "max"
+no_data_state: "OK"
+exec_err_state: "Error"
+labels:
+  service: "postgres"
+  severity: "warning"
+  team: "infra"
+summary: |
+  PostgreSQL: {{ printf "%.0f" $values.B.Value }}% соединений занято на {{ $labels.instance }}
+description: |
+  На {{ $labels.instance }} занято {{ printf "%.0f" $values.B.Value }}% от max_connections PostgreSQL.
+  При достижении лимита новые подключения будут отклоняться с ошибкой.