feat(relayer): adding a prometheus guage for relayer balance

packages/relayer/pkg/mock/eth_client.go

@@ -142,3 +142,10 @@ func (c *EthClient) SubscribeNewHead(ctx context.Context, ch chan<- *types.Heade
 func (c *EthClient) TransactionSender(ctx context.Context, tx *types.Transaction, blockHash common.Hash, txIndex uint) (common.Address, error) {
 	return common.Address{}, nil
 }
+
+func (c *EthClient) BalanceAt(
+	ctx context.Context,
+	account common.Address,
+	blockNumber *big.Int) (*big.Int, error) {
+	return big.NewInt(100), nil
+}

packages/relayer/processor/process_message.go

@@ -6,6 +6,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"log/slog"
+	"math"
 	"math/big"
 	"strings"
 	"time"
@@ -394,6 +395,8 @@ func (p *Processor) sendProcessMessageCall(
 	event *bridge.BridgeMessageSent,
 	proof []byte,
 ) (*types.Receipt, error) {
+	defer p.logRelayerBalance(ctx)
+
 	received, err := p.destBridge.IsMessageReceived(nil, event.Message, proof)
 	if err != nil {
 		return nil, err
@@ -447,9 +450,8 @@ func (p *Processor) sendProcessMessageCall(
 
 			return nil, relayer.ErrUnprofitable
 		}
-		// now simulate the transaction and lets confirm
-		// it is profitable
 
+		// now simulate the transaction and lets confirm it is profitable
 		auth, err := bind.NewKeyedTransactorWithChainID(p.ecdsaKey, p.destChainId)
 		if err != nil {
 			return nil, err
@@ -497,7 +499,6 @@ func (p *Processor) sendProcessMessageCall(
 		uint64(event.Message.GasLimit),
 	) {
 		slog.Error("can not process message after waiting for confirmations", "err", errUnprocessable)
-
 		return nil, errUnprocessable
 	}
 
@@ -553,6 +554,29 @@ func (p *Processor) sendProcessMessageCall(
 	return receipt, nil
 }
 
+// retrieve the balance of the relayer and set Prometheus
+func (p *Processor) logRelayerBalance(ctx context.Context) {
+	balance, err := p.destEthClient.BalanceAt(ctx, p.relayerAddr, nil)
+	if err != nil {
+		slog.Warn("Failed to retrieve relayer balance", "error", err)
+		return
+	}
+
+	balanceFloat := new(big.Float).SetInt(balance)
+	balanceEth := new(big.Float).Quo(
+		balanceFloat,
+		big.NewFloat(math.Pow10(18)),
+	)
+
+	slog.Info("Relayer balance",
+		"relayerAddress", p.relayerAddr,
+		"balance", balanceEth.Text('f', 18),
+	)
+
+	balanceEthFloat, _ := balanceEth.Float64()
+	relayer.RelayerKeyBalanceGauge.Set(balanceEthFloat)
+}
+
 // saveMessageStatusChangedEvent writes the MessageStatusChanged event to the
 // database after a message is processed
 func (p *Processor) saveMessageStatusChangedEvent(

packages/relayer/processor/processor.go

@@ -60,6 +60,7 @@ type ethClient interface {
 	ChainID(ctx context.Context) (*big.Int, error)
 	SubscribeNewHead(ctx context.Context, ch chan<- *types.Header) (ethereum.Subscription, error)
 	EstimateGas(ctx context.Context, msg ethereum.CallMsg) (uint64, error)
+	BalanceAt(ctx context.Context, account common.Address, blockNumber *big.Int) (*big.Int, error)
 }
 
 // hop is a struct which needs to be created based on the config parameters

packages/relayer/prometheus.go

@@ -146,4 +146,8 @@ var (
 		Name: "message_processed_events_after_retry_error_count",
 		Help: "The total number of errors logged for MessageProcessed events after retries",
 	})
+	RelayerKeyBalanceGauge = promauto.NewGauge(prometheus.GaugeOpts{
+		Name: "relayer_key_balance",
+		Help: "Current balance of the relayer key",
+	})
 )