chore: convert agent stats to use a table (#6374)

* chore: convert workspace agent stats from json to table

* chore: convert agent stats to use a table

Backwards compatibility becomes hard when all agent stats are in a JSON blob.
We also want to query this table for new agents that are failing health checks
so we can display it in the UI.

* Fix migration using default values

coderd/workspaceagents_test.go

@@ -1178,12 +1178,12 @@ func TestWorkspaceAgentReportStats(t *testing.T) {
 		agentClient.SetSessionToken(authToken)
 
 		_, err := agentClient.PostStats(context.Background(), &agentsdk.Stats{
-			ConnsByProto: map[string]int64{"TCP": 1},
-			NumConns:     1,
-			RxPackets:    1,
-			RxBytes:      1,
-			TxPackets:    1,
-			TxBytes:      1,
+			ConnectionsByProto: map[string]int64{"TCP": 1},
+			ConnectionCount:    1,
+			RxPackets:          1,
+			RxBytes:            1,
+			TxPackets:          1,
+			TxBytes:            1,
 		})
 		require.NoError(t, err)