chore: replace GetManagedAgentCount query with aggregate table (#19636)

- Removes GetManagedAgentCount query
- Adds new table `usage_events_daily` which stores aggregated usage
events by the type and UTC day
- Adds trigger to update the values in this table when a new row is
inserted into `usage_events`
- Adds a migration that adds `usage_events_daily` rows for existing data
in `usage_events`
- Adds tests for the trigger
- Adds tests for the backfill query in the migration

Since the `usage_events` table is unreleased currently, this migration
will do nothing on real deployments and will only affect preview
deployments such as dogfood.

Closes https://github.com/coder/internal/issues/943

coderd/database/migrations/000362_aggregate_usage_events.up.sql

@@ -0,0 +1,65 @@
+CREATE TABLE usage_events_daily (
+  day date NOT NULL, -- always grouped by day in UTC
+  event_type text NOT NULL,
+  usage_data jsonb NOT NULL,
+  PRIMARY KEY (day, event_type)
+);
+
+COMMENT ON TABLE usage_events_daily IS 'usage_events_daily is a daily rollup of usage events. It stores the total usage for each event type by day.';
+COMMENT ON COLUMN usage_events_daily.day IS 'The date of the summed usage events, always in UTC.';
+
+-- Function to handle usage event aggregation
+CREATE OR REPLACE FUNCTION aggregate_usage_event()
+RETURNS TRIGGER AS $$
+BEGIN
+    -- Check for supported event types and throw error for unknown types
+    IF NEW.event_type NOT IN ('dc_managed_agents_v1') THEN
+        RAISE EXCEPTION 'Unhandled usage event type in aggregate_usage_event: %', NEW.event_type;
+    END IF;
+
+    INSERT INTO usage_events_daily (day, event_type, usage_data)
+    VALUES (
+        -- Extract the date from the created_at timestamp, always using UTC for
+        -- consistency
+        date_trunc('day', NEW.created_at AT TIME ZONE 'UTC')::date,
+        NEW.event_type,
+        NEW.event_data
+    )
+    ON CONFLICT (day, event_type) DO UPDATE SET
+        usage_data = CASE
+            -- Handle simple counter events by summing the count
+            WHEN NEW.event_type IN ('dc_managed_agents_v1') THEN
+                jsonb_build_object(
+                    'count',
+                    COALESCE((usage_events_daily.usage_data->>'count')::bigint, 0) +
+                    COALESCE((NEW.event_data->>'count')::bigint, 0)
+                )
+        END;
+
+    RETURN NEW;
+END;
+$$ LANGUAGE plpgsql;
+
+-- Create trigger to automatically aggregate usage events
+CREATE TRIGGER trigger_aggregate_usage_event
+    AFTER INSERT ON usage_events
+    FOR EACH ROW
+    EXECUTE FUNCTION aggregate_usage_event();
+
+-- Populate usage_events_daily with existing data
+INSERT INTO
+    usage_events_daily (day, event_type, usage_data)
+SELECT
+    date_trunc('day', created_at AT TIME ZONE 'UTC')::date AS day,
+    event_type,
+    jsonb_build_object('count', SUM((event_data->>'count')::bigint)) AS usage_data
+FROM
+    usage_events
+WHERE
+    -- The only event type we currently support is dc_managed_agents_v1
+    event_type = 'dc_managed_agents_v1'
+GROUP BY
+    date_trunc('day', created_at AT TIME ZONE 'UTC')::date,
+    event_type
+ON CONFLICT (day, event_type) DO UPDATE SET
+    usage_data = EXCLUDED.usage_data;