les, les/lespay/client: add service value statistics and API (#20837)

This PR adds service value measurement statistics to the light client. It
also adds a private API that makes these statistics accessible. A follow-up
PR will add the new server pool which uses these statistics to select
servers with good performance.

This document describes the function of the new components:
https://gist.github.com/zsfelfoldi/3c7ace895234b7b345ab4f71dab102d4

Co-authored-by: rjl493456442 <garyrong0905@gmail.com>
Co-authored-by: rjl493456442 <garyrong0905@gmail.com>

les/txrelay.go

@@ -144,7 +144,7 @@ func (ltrx *lesTxRelay) send(txs types.Transactions, count int) {
 				peer := dp.(*serverPeer)
 				cost := peer.getTxRelayCost(len(ll), len(enc))
 				peer.fcServer.QueuedRequest(reqID, cost)
-				return func() { peer.sendTxs(reqID, enc) }
+				return func() { peer.sendTxs(reqID, len(ll), enc) }
 			},
 		}
 		go ltrx.retriever.retrieve(context.Background(), reqID, rq, func(p distPeer, msg *Msg) error { return nil }, ltrx.stop)