perf: don't call GetUserByID unnecessarily for Agents metrics loops (#19395)

At the moment, the loop which retrieves and updates the values of the
agents metrics excessively calls `GetUserByID` (a DB query). First it
retrieves a list of all workspaces, filtering out inactive agents (not
entirely clear to me whether this is non-running workspaces, or just
dead agents), and then iterates over those workspaces to get the rest of
the relevant data for the metrics. The next call is `GetUserByID` for
`workspace.OwnerID`. This is unnecessary because the `workspaces_visible` view we pull workspaces from has already been joined with the users table to get the username/name/etc.

This should at least partially resolve
https://github.com/coder/internal/issues/726 
---------

Signed-off-by: Callum Styan <callumstyan@gmail.com>

coderd/agentapi/stats_test.go

@@ -41,11 +41,12 @@ func TestUpdateStates(t *testing.T) {
 			Name: "tpl",
 		}
 		workspace = database.Workspace{
-			ID:           uuid.New(),
-			OwnerID:      user.ID,
-			TemplateID:   template.ID,
-			Name:         "xyz",
-			TemplateName: template.Name,
+			ID:            uuid.New(),
+			OwnerID:       user.ID,
+			OwnerUsername: user.Username,
+			TemplateID:    template.ID,
+			Name:          "xyz",
+			TemplateName:  template.Name,
 		}
 		agent = database.WorkspaceAgent{
 			ID:   uuid.New(),
@@ -138,9 +139,6 @@ func TestUpdateStates(t *testing.T) {
 		// Workspace gets fetched.
 		dbM.EXPECT().GetWorkspaceByAgentID(gomock.Any(), agent.ID).Return(workspace, nil)
 
-		// User gets fetched to hit the UpdateAgentMetricsFn.
-		dbM.EXPECT().GetUserByID(gomock.Any(), user.ID).Return(user, nil)
-
 		// We expect an activity bump because ConnectionCount > 0.
 		dbM.EXPECT().ActivityBumpWorkspace(gomock.Any(), database.ActivityBumpWorkspaceParams{
 			WorkspaceID:   workspace.ID,
@@ -380,9 +378,6 @@ func TestUpdateStates(t *testing.T) {
 			LastUsedAt: now.UTC(),
 		}).Return(nil)
 
-		// User gets fetched to hit the UpdateAgentMetricsFn.
-		dbM.EXPECT().GetUserByID(gomock.Any(), user.ID).Return(user, nil)
-
 		resp, err := api.UpdateStats(context.Background(), req)
 		require.NoError(t, err)
 		require.Equal(t, &agentproto.UpdateStatsResponse{
@@ -498,9 +493,6 @@ func TestUpdateStates(t *testing.T) {
 			LastUsedAt: now,
 		}).Return(nil)
 
-		// User gets fetched to hit the UpdateAgentMetricsFn.
-		dbM.EXPECT().GetUserByID(gomock.Any(), user.ID).Return(user, nil)
-
 		// Ensure that pubsub notifications are sent.
 		notifyDescription := make(chan struct{})
 		ps.SubscribeWithErr(wspubsub.WorkspaceEventChannel(workspace.OwnerID),