feat: only log batch tx size once

Previously it was being logged every time we modified the gas price.
Additionally, we log the raw byte length rather than tx.Size(). The
latter returns a float value of type StorageSize, which explains why the
metrics haven't been indicating actual byte values.

go/batch-submitter/service.go

 package batchsubmitter
 
 import (
+	"bytes"
 	"context"
 	"math/big"
 	"sync"
@@ -183,6 +184,14 @@ func (s *Service) eventLoop() {
 			batchTxBuildTime := time.Since(batchTxBuildStart) / time.Millisecond
 			s.metrics.BatchTxBuildTime.Set(float64(batchTxBuildTime))
 
+			// Record the size of the batch transaction.
+			var txBuf bytes.Buffer
+			if err := tx.EncodeRLP(&txBuf); err != nil {
+				log.Error(name+" unable to encode batch tx", "err", err)
+				continue
+			}
+			s.metrics.BatchSizeInBytes.Observe(float64(len(txBuf.Bytes())))
+
 			// Construct the transaction submission clousure that will attempt
 			// to send the next transaction at the given nonce and gas price.
 			sendTx := func(
@@ -207,8 +216,6 @@ func (s *Service) eventLoop() {
 					"gasPrice", gasPrice,
 				)
 
-				s.metrics.BatchSizeInBytes.Observe(float64(tx.Size()))
-
 				return tx, nil
 			}