controlclient: log the entire netmap up to every 5 minutes.

We log it once upon receiving the first copy of the map, then
subsequently when a new one appears, but only if we haven't logged one
less than 5 minutes ago.

This avoids overly cluttering the log (as we did before, logging the
netmap every time one appeared, which could be hundreds of lines every
few seconds), but still gives the log enough context to help in
diagnosing problems retroactively.

control/controlclient/direct.go

@@ -76,6 +76,7 @@ type Direct struct {
 	httpc           *http.Client // HTTP client used to talk to tailcontrol
 	serverURL       string       // URL of the tailcontrol server
 	timeNow         func() time.Time
+	lastPrintMap    time.Time
 	newDecompressor func() (Decompressor, error)
 	keepAlive       bool
 	logf            logger.Logf
@@ -579,7 +580,17 @@ func (c *Direct) PollNetMap(ctx context.Context, maxPolls int, cb func(*NetworkM
 		} else {
 			nm.MachineStatus = tailcfg.MachineUnauthorized
 		}
-		//c.logf("new network map[%d]:\n%s", i, nm.Concise())
+
+		// Printing the netmap can be extremely verbose, but is very
+		// handy for debugging. Let's limit how often we do it.
+		// Code elsewhere prints netmap diffs every time, so this
+		// occasional full dump, plus incremental diffs, should do
+		// the job.
+		now := c.timeNow()
+		if now.Sub(c.lastPrintMap) >= 5*time.Minute {
+			c.lastPrintMap = now
+			c.logf("new network map[%d]:\n%s", i, nm.Concise())
+		}
 
 		c.mu.Lock()
 		c.expiry = &nm.Expiry