net/connstats: enforce maximum number of connections (#6760)

The Tailscale logging service has a hard limit on the maximum
log message size that can be accepted.
We want to ensure that netlog messages never exceed
this limit otherwise a client cannot transmit logs.

Move the goroutine for periodically dumping netlog messages
from wgengine/netlog to net/connstats.
This allows net/connstats to manage when it dumps messages,
either based on time or by size.

Updates tailscale/corp#8427

Signed-off-by: Joe Tsai <joetsai@digital-static.net>

types/netlogtype/netlogtype.go

@@ -30,6 +30,15 @@ type Message struct {
 }
 
 const (
+	messageJSON      = `{"nodeId":"n0123456789abcdefCNTRL",` + maxJSONTimeRange + `,` + minJSONTraffic + `}`
+	maxJSONTimeRange = `"start":` + maxJSONRFC3339 + `,"end":` + maxJSONRFC3339
+	maxJSONRFC3339   = `"0001-01-01T00:00:00.000000000Z"`
+	minJSONTraffic   = `"virtualTraffic":{},"subnetTraffic":{},"exitTraffic":{},"physicalTraffic":{}`
+
+	// MaxMessageJSONSize is the overhead size of Message when it is
+	// serialized as JSON assuming that each traffic map is populated.
+	MaxMessageJSONSize = len(messageJSON)
+
 	maxJSONConnCounts = `{` + maxJSONConn + `,` + maxJSONCounts + `}`
 	maxJSONConn       = `"proto":` + maxJSONProto + `,"src":` + maxJSONAddrPort + `,"dst":` + maxJSONAddrPort
 	maxJSONProto      = `255`