fix: stop checking gauges unrelated to TestAgent_Stats_Magic (#17290)

Fixes https://github.com/coder/internal/issues/564

The test is asserting too much, including stats guages that are not directly related to the thing we are trying to test: ConnectionCount, RxBytes, and TxBytes.  I think the author assumed that these are counts that only go up, but they are guages and eventually zero back out, so there are race condtions where not all of them are non-zero at the same time.

agent/agent_test.go

@@ -190,7 +190,7 @@ func TestAgent_Stats_Magic(t *testing.T) {
 			s, ok := <-stats
 			t.Logf("got stats: ok=%t, ConnectionCount=%d, RxBytes=%d, TxBytes=%d, SessionCountVSCode=%d, ConnectionMedianLatencyMS=%f",
 				ok, s.ConnectionCount, s.RxBytes, s.TxBytes, s.SessionCountVscode, s.ConnectionMedianLatencyMs)
-			return ok && s.ConnectionCount > 0 && s.RxBytes > 0 && s.TxBytes > 0 &&
+			return ok &&
 				// Ensure that the connection didn't count as a "normal" SSH session.
 				// This was a special one, so it should be labeled specially in the stats!
 				s.SessionCountVscode == 1 &&
@@ -258,8 +258,7 @@ func TestAgent_Stats_Magic(t *testing.T) {
 			s, ok := <-stats
 			t.Logf("got stats with conn open: ok=%t, ConnectionCount=%d, SessionCountJetBrains=%d",
 				ok, s.ConnectionCount, s.SessionCountJetbrains)
-			return ok && s.ConnectionCount > 0 &&
-				s.SessionCountJetbrains == 1
+			return ok && s.SessionCountJetbrains == 1
 		}, testutil.WaitLong, testutil.IntervalFast,
 			"never saw stats with conn open",
 		)