derp: track client-advertised non-ideal DERP connections in more places

In f77821fd63 (released in v1.72.0), we made the client tell a DERP server
when the connection was not its ideal choice (the first node in its region).

But we didn't do anything with that information until now. This adds a
metric about how many such connections are on a given derper, and also
adds a bit to the PeerPresentFlags bitmask so watchers can identify
(and rebalance) them.

Updates tailscale/corp#372

Change-Id: Ief8af448750aa6d598e5939a57c062f4e55962be
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

derp/derphttp/derphttp_client.go

@@ -498,7 +498,7 @@ func (c *Client) connect(ctx context.Context, caller string) (client *derp.Clien
 	req.Header.Set("Connection", "Upgrade")
 	if !idealNodeInRegion && reg != nil {
 		// This is purely informative for now (2024-07-06) for stats:
-		req.Header.Set("Ideal-Node", reg.Nodes[0].Name)
+		req.Header.Set(derp.IdealNodeHeader, reg.Nodes[0].Name)
 		// TODO(bradfitz,raggi): start a time.AfterFunc for 30m-1h or so to
 		// dialNode(reg.Nodes[0]) and see if we can even TCP connect to it. If
 		// so, TLS handshake it as well (which is mixed up in this massive