FEATURE: Export metric for highest sequence usage

Plugin already exports `postgres_highest_sequence`, but the value cannot
be reliable used for alerts because it is not linked with the maximum
value of the column that uses it.

In other words, a sequence may be used with an `integer` or a `bigint`
column, which have very different maximum values. The highest
`last_value` reported may be cause of concern for an `integer` column
because it is close to the limit, but not a problem for a `bigint`
column.

lib/collector.rb

@@ -185,6 +185,11 @@ def ensure_global_metrics
         "The highest last_value from the pg_sequences table",
       )
 
+      global_metrics << Gauge.new(
+        "postgres_highest_sequence_usage",
+        "The highest usage of the sequences. Usage is calculated as the ratio between last_value from the pg_sequences table and the determined maximum value for the column using the sequence",
+      )
+
       @global_metrics = global_metrics
     end
 

lib/internal_metric/global.rb

@@ -27,7 +27,8 @@ class Global < Base
               :missing_s3_uploads,
               :version_info,
               :readonly_sites,
-              :postgres_highest_sequence
+              :postgres_highest_sequence,
+              :postgres_highest_sequence_usage
 
     def initialize
       @active_app_reqs = 0
@@ -168,13 +169,17 @@ def collect
       @readonly_sites = collect_readonly_sites
 
       @postgres_highest_sequence = calc_postgres_highest_sequence
+
+      @postgres_highest_sequence_usage = calc_postgres_highest_sequence_usage
     end
 
     # For testing purposes
     def reset!
       @@missing_uploads = nil
       @@postgres_highest_sequence_last_check = nil
       @@postgres_highest_sequence_cache = nil
+      @@postgres_highest_sequence_usage_last_check = nil
+      @@postgres_highest_sequence_usage_cache = nil
     end
 
     private
@@ -381,5 +386,46 @@ def calc_postgres_highest_sequence
         Discourse.warn_exception(e, message: "Failed to collect postgres_highest_sequence value")
       end
     end
+
+    def calc_postgres_highest_sequence_usage
+      @@postgres_highest_sequence_usage_last_check ||= 0
+
+      if @@postgres_highest_sequence_usage_last_check >=
+           Time.now.to_i - PG_HIGHEST_SEQUENCE_CHECK_SECONDS
+        return @@postgres_highest_sequence_usage_cache
+      end
+
+      @@postgres_highest_sequence_usage_last_check = Time.now.to_i
+
+      result = {}
+
+      RailsMultisite::ConnectionManagement.each_connection do |db|
+        result[{ db: db }] = DB.query_single(<<~SQL)[0]
+          WITH sequences AS (
+              SELECT table_name,
+                     column_name,
+                     information_schema.columns.data_type,
+                     COALESCE(last_value, 0) last_value,
+                     CASE information_schema.columns.data_type
+                         WHEN 'integer' THEN POWER(2, 31)
+                         WHEN 'bigint' THEN POWER(2, 63)
+                         ELSE NULL
+                     END max_value
+              FROM information_schema.columns
+              JOIN pg_sequences ON sequencename = REPLACE(REPLACE(column_default, 'nextval(''', ''), '''::regclass)', '')
+          )
+          SELECT MAX(last_value / max_value) FROM sequences
+        SQL
+      end
+
+      @@postgres_highest_sequence_usage_cache = result
+    rescue => e
+      if @postgres_master_available == 1
+        Discourse.warn_exception(
+          e,
+          message: "Failed to collect postgres_highest_sequence_usage value",
+        )
+      end
+    end
   end
 end

spec/lib/internal_metric/global_spec.rb

@@ -113,8 +113,12 @@ module DiscoursePrometheus::InternalMetric
       expect(metric.postgres_highest_sequence).to be_a_kind_of(Hash)
       expect(metric.postgres_highest_sequence[{ db: "default" }]).to be_present
 
+      expect(metric.postgres_highest_sequence_usage).to be_a_kind_of(Hash)
+      expect(metric.postgres_highest_sequence_usage[{ db: "default" }]).to be_present
+
       expect do metric.collect end.not_to change {
         metric.class.class_variable_get(:@@postgres_highest_sequence_last_check)
+        metric.class.class_variable_get(:@@postgres_highest_sequence_usage_last_check)
       }
     end
   end