p2p: track peers stored on startup (#8787) (#8788)

(cherry picked from commit 0ac03468d8)

Co-authored-by: Sam Kleinman <garen@tychoish.com>

internal/p2p/peermanager.go

@@ -367,6 +367,9 @@ func (m *PeerManager) configurePeers() error {
 			}
 		}
 	}
+
+	m.metrics.PeersStored.Add(float64(m.store.Size()))
+
 	return nil
 }