ipn/ipnlocal,wgengine/magicsock: add basic counters for cached peer connectivity (#19699)

Add new clientmetric counters for establishing contact with peers while using
cached network map data. To do this, instrument the magicsock.Conn with a bit
to indicate whether its peer data came from a cached netmap. If so, there are
two conditions we will count as establishing connectivity to a peer:

  - Receipt of a CallMeMaybe from a peer via disco.
  - Establishing a valid endpoint address for a peer.

In vmtest, add Env.ClientMetrics to scrape metrics from the specified node.
Use this to check that counters were updated in caching tests.

Updates https://github.com/tailscale/projects/issues/13
Updates #12639

Change-Id: Ie8cf3244ac8af4f5bcfe4d0d944078da2ba08990
Signed-off-by: M. J. Fromberger <fromberger@tailscale.com>

wgengine/magicsock/endpoint.go

@@ -138,6 +138,13 @@ func (de *endpoint) udpRelayEndpointReady(maybeBest addrQuality) {
 func (de *endpoint) setBestAddrLocked(v addrQuality) {
 	if v.epAddr != de.bestAddr.epAddr {
 		de.probeUDPLifetime.resetCycleEndpointLocked()
+
+		// Reaching here, if we are using data from a cached netmap and we are
+		// upgrading from an invalid (missing) address to a valid one, increment
+		// the counter for peers established.
+		if !de.bestAddr.ap.IsValid() && v.ap.IsValid() && de.c.usingCachedNetmap.Load() {
+			metricCachedPeerContactDirect.Add(1)
+		}
 	}
 	de.bestAddr = v
 }