chore: track the first time html is served in telemetry (#16334)

Addresses https://github.com/coder/nexus/issues/175.

## Changes

- Adds the `telemetry_items` database table. It's a key value store for
telemetry events that don't fit any other database tables.
- Adds a telemetry report when HTML is served for the first time in
`site.go`.

coderd/telemetry/telemetry.go

@@ -579,6 +579,17 @@ func (r *remoteReporter) createSnapshot() (*Snapshot, error) {
 		}
 		return nil
 	})
+	eg.Go(func() error {
+		items, err := r.options.Database.GetTelemetryItems(ctx)
+		if err != nil {
+			return xerrors.Errorf("get telemetry items: %w", err)
+		}
+		snapshot.TelemetryItems = make([]TelemetryItem, 0, len(items))
+		for _, item := range items {
+			snapshot.TelemetryItems = append(snapshot.TelemetryItems, ConvertTelemetryItem(item))
+		}
+		return nil
+	})
 
 	err := eg.Wait()
 	if err != nil {
@@ -985,6 +996,15 @@ func ConvertOrganization(org database.Organization) Organization {
 	}
 }
 
+func ConvertTelemetryItem(item database.TelemetryItem) TelemetryItem {
+	return TelemetryItem{
+		Key:       item.Key,
+		Value:     item.Value,
+		CreatedAt: item.CreatedAt,
+		UpdatedAt: item.UpdatedAt,
+	}
+}
+
 // Snapshot represents a point-in-time anonymized database dump.
 // Data is aggregated by latest on the server-side, so partial data
 // can be sent without issue.
@@ -1012,6 +1032,7 @@ type Snapshot struct {
 	Workspaces                []Workspace                 `json:"workspaces"`
 	NetworkEvents             []NetworkEvent              `json:"network_events"`
 	Organizations             []Organization              `json:"organizations"`
+	TelemetryItems            []TelemetryItem             `json:"telemetry_items"`
 }
 
 // Deployment contains information about the host running Coder.
@@ -1536,6 +1557,25 @@ type Organization struct {
 	CreatedAt time.Time `json:"created_at"`
 }
 
+type telemetryItemKey string
+
+// The comment below gets rid of the warning that the name "TelemetryItemKey" has
+// the "Telemetry" prefix, and that stutters when you use it outside the package
+// (telemetry.TelemetryItemKey...). "TelemetryItem" is the name of a database table,
+// so it makes sense to use the "Telemetry" prefix.
+//
+//revive:disable:exported
+const (
+	TelemetryItemKeyHTMLFirstServedAt telemetryItemKey = "html_first_served_at"
+)
+
+type TelemetryItem struct {
+	Key       string    `json:"key"`
+	Value     string    `json:"value"`
+	CreatedAt time.Time `json:"created_at"`
+	UpdatedAt time.Time `json:"updated_at"`
+}
+
 type noopReporter struct{}
 
 func (*noopReporter) Report(_ *Snapshot) {}

coderd/telemetry/telemetry_test.go

@@ -75,6 +75,10 @@ func TestTelemetry(t *testing.T) {
 			Health:       database.WorkspaceAppHealthDisabled,
 			OpenIn:       database.WorkspaceAppOpenInSlimWindow,
 		})
+		_ = dbgen.TelemetryItem(t, db, database.TelemetryItem{
+			Key:   string(telemetry.TelemetryItemKeyHTMLFirstServedAt),
+			Value: time.Now().Format(time.RFC3339),
+		})
 		group := dbgen.Group(t, db, database.Group{})
 		_ = dbgen.GroupMember(t, db, database.GroupMemberTable{UserID: user.ID, GroupID: group.ID})
 		wsagent := dbgen.WorkspaceAgent(t, db, database.WorkspaceAgent{})
@@ -127,7 +131,7 @@ func TestTelemetry(t *testing.T) {
 		require.Len(t, snapshot.WorkspaceProxies, 1)
 		require.Len(t, snapshot.WorkspaceModules, 1)
 		require.Len(t, snapshot.Organizations, 1)
-
+		require.Len(t, snapshot.TelemetryItems, 1)
 		wsa := snapshot.WorkspaceAgents[0]
 		require.Len(t, wsa.Subsystems, 2)
 		require.Equal(t, string(database.WorkspaceAgentSubsystemEnvbox), wsa.Subsystems[0])
@@ -316,6 +320,47 @@ func TestTelemetryInstallSource(t *testing.T) {
 	require.Equal(t, "aws_marketplace", deployment.InstallSource)
 }
 
+func TestTelemetryItem(t *testing.T) {
+	t.Parallel()
+	ctx := testutil.Context(t, testutil.WaitMedium)
+	db, _ := dbtestutil.NewDB(t)
+	key := testutil.GetRandomName(t)
+	value := time.Now().Format(time.RFC3339)
+
+	err := db.InsertTelemetryItemIfNotExists(ctx, database.InsertTelemetryItemIfNotExistsParams{
+		Key:   key,
+		Value: value,
+	})
+	require.NoError(t, err)
+
+	item, err := db.GetTelemetryItem(ctx, key)
+	require.NoError(t, err)
+	require.Equal(t, item.Key, key)
+	require.Equal(t, item.Value, value)
+
+	// Inserting a new value should not update the existing value
+	err = db.InsertTelemetryItemIfNotExists(ctx, database.InsertTelemetryItemIfNotExistsParams{
+		Key:   key,
+		Value: "new_value",
+	})
+	require.NoError(t, err)
+
+	item, err = db.GetTelemetryItem(ctx, key)
+	require.NoError(t, err)
+	require.Equal(t, item.Value, value)
+
+	// Upserting a new value should update the existing value
+	err = db.UpsertTelemetryItem(ctx, database.UpsertTelemetryItemParams{
+		Key:   key,
+		Value: "new_value",
+	})
+	require.NoError(t, err)
+
+	item, err = db.GetTelemetryItem(ctx, key)
+	require.NoError(t, err)
+	require.Equal(t, item.Value, "new_value")
+}
+
 func collectSnapshot(t *testing.T, db database.Store, addOptionsFn func(opts telemetry.Options) telemetry.Options) (*telemetry.Deployment, *telemetry.Snapshot) {
 	t.Helper()
 	deployment := make(chan *telemetry.Deployment, 64)