PostgreSQL observ lib (#1565)

* draft

* add settings table

* some other improvements

* add cluster dashboard

* some other improvements

* fix queries dashboard

* cleanup prolixive comments

* small fixes

* lint

* add mixtool

* lint

* add lint exclusions

* add lint exclusions

* grr pattern

* several improvements

* rename instance dashboard and clean orphaned signals and panels

* small improv

* .lint update

* other improvements

* lint

* refactor table queries to use queriesSelector

* correct threshold colors

* lint

* formatting

* more name formatting

* lint

* format table

* fix signal

Author: gaantunes

postgres-observ-lib/.go-version

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+1.25.1

postgres-observ-lib/.lint

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+exclusions:
+  panel-datasource-rule:
+    reason: "grafonnet created panels use --Mixed-- datasource by default"
+
+  template-instance-rule:
+    reason: "Instance selector is single-select for instance-specific dashboards; Cluster dashboard is cluster-wide"
+    entries:
+      - dashboard: "PostgreSQL cluster overview"
+      - dashboard: "PostgreSQL instance overview"
+      - dashboard: "PostgreSQL query performance"
+
+  target-instance-rule:
+    reason: "Cluster level dashboard shows all instances"
+    entries:
+      - dashboard: "PostgreSQL cluster overview"
+
+  panel-units-rule:
+    reason: "rows/s is semantically correct; table panels don't require units"
+    entries:
+      - panel: "Write operations (primary)"
+      - panel: "Read operations by instance"
+      - panel: "Rows activity"
+      - panel: "Top queries by rows"
+      - panel: "Unused indexes"
+      - panel: "PostgreSQL settings"
+      - panel: "Read/Write over time"

postgres-observ-lib/Makefile

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+include ../Makefile_mixin