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>

ipn/ipnlocal/local.go

@@ -6587,7 +6587,11 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	b.currentNode().SetNetMap(nm)
 	if ms, ok := b.sys.MagicSock.GetOK(); ok {
 		if nm != nil {
-			ms.SetNetworkMap(nm.SelfNode, nm.Peers)
+			if nm.Cached {
+				ms.SetNetworkMapCached(nm.SelfNode, nm.Peers)
+			} else {
+				ms.SetNetworkMap(nm.SelfNode, nm.Peers)
+			}
 		} else {
 			ms.SetNetworkMap(tailcfg.NodeView{}, nil)
 		}