Daily Active User Metrics (#3735)

* agent: add StatsReporter

* Stabilize protoc

coderd/database/migrations/000042_agent_stats.down.sql

@@ -0,0 +1 @@
+DROP TABLE agent_stats;

coderd/database/migrations/000042_agent_stats.up.sql

@@ -0,0 +1,16 @@
+CREATE TABLE agent_stats (
+    id uuid NOT NULL,
+    PRIMARY KEY (id),
+    created_at timestamptz NOT NULL,
+    user_id uuid NOT NULL,
+    agent_id uuid NOT NULL,
+    workspace_id uuid NOT NULL,
+    template_id uuid NOT NULL,
+    payload jsonb NOT NULL
+);
+
+-- We use created_at for DAU analysis and pruning.
+CREATE INDEX idx_agent_stats_created_at ON agent_stats USING btree (created_at);
+
+-- We perform user grouping to analyze DAUs.
+CREATE INDEX idx_agent_stats_user_id ON agent_stats USING btree (user_id);