op-service: Provide balance metrics utility

This function periodically records the balance of the supplied
account through metrics. The balance is recorded in Ether (not wei)
to stay in bounds of a float64.

op-service/go.sum

@@ -102,6 +102,7 @@ github.com/dustin/go-humanize v0.0.0-20171111073723-bb3d318650d4/go.mod h1:Htrtb
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@@ -119,6 +120,7 @@ github.com/fsnotify/fsnotify v1.4.7/go.mod h1:jwhsz4b93w/PPRr/qN1Yymfu8t87LnFCMo
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@@ -205,6 +207,7 @@ github.com/google/pprof v0.0.0-20200708004538-1a94d8640e99/go.mod h1:ZgVRPoUq/hf
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+github.com/google/uuid v1.2.0 h1:qJYtXnJRWmpe7m/3XlyhrsLrEURqHRM2kxzoxXqyUDs=
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@@ -241,6 +244,8 @@ github.com/hashicorp/logutils v1.0.0/go.mod h1:QIAnNjmIWmVIIkWDTG1z5v++HQmx9WQRO
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+github.com/holiman/bloomfilter/v2 v2.0.3 h1:73e0e/V0tCydx14a0SCYS/EWCxgwLZ18CZcZKVu0fao=
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@@ -406,6 +411,7 @@ github.com/prometheus/tsdb v0.10.0/go.mod h1:oi49uRhEe9dPUTlS3JRZOwJuVi6tmh10QSg
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@@ -432,6 +438,7 @@ github.com/sony/gobreaker v0.4.1/go.mod h1:ZKptC7FHNvhBz7dN2LGjPVBz2sZJmc0/PkyDJ
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@@ -454,6 +461,7 @@ github.com/tklauser/go-sysconf v0.3.10/go.mod h1:C8XykCvCb+Gn0oNCWPIlcb0RuglQTYa
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@@ -661,6 +669,7 @@ golang.org/x/time v0.0.0-20180412165947-fbb02b2291d2/go.mod h1:tRJNPiyCQ0inRvYxb
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op-service/metrics/balance.go

+package metrics
+
+import (
+	"context"
+	"math/big"
+	"time"
+
+	"github.com/ethereum/go-ethereum/common"
+	"github.com/ethereum/go-ethereum/ethclient"
+	"github.com/ethereum/go-ethereum/log"
+	"github.com/ethereum/go-ethereum/params"
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/promauto"
+)
+
+// weiToEther divides the wei value by 10^18 to get a number in ether as a float64
+func weiToEther(wei *big.Int) float64 {
+	num := new(big.Rat).SetInt(wei)
+	denom := big.NewRat(params.Ether, 1)
+	num = num.Quo(num, denom)
+	f, _ := num.Float64()
+	return f
+}
+
+// LaunchBalanceMetrics fires off a go rountine that queries the balance of the supplied account & periodically records it
+// to the balance metric of the namespace. The balance of the account is recorded in Ether (not Wei).
+// Cancel the supplied context to shut down the go routine
+func LaunchBalanceMetrics(ctx context.Context, log log.Logger, r *prometheus.Registry, ns string, client *ethclient.Client, account common.Address) {
+	go func() {
+		balanceGuage := promauto.With(r).NewGauge(prometheus.GaugeOpts{
+			Namespace: ns,
+			Name:      "balance",
+			Help:      "balance (in ether) of account " + account.String(),
+		})
+
+		ticker := time.NewTicker(10 * time.Second)
+		defer ticker.Stop()
+
+		for {
+			select {
+			case <-ticker.C:
+				ctx, cancel := context.WithTimeout(ctx, 1*time.Minute)
+				bigBal, err := client.BalanceAt(ctx, account, nil)
+				if err != nil {
+					log.Warn("failed to get balance of account", "err", err, "address", account)
+					cancel()
+					continue
+				}
+				bal := weiToEther(bigBal)
+				balanceGuage.Set(bal)
+				cancel()
+			case <-ctx.Done():
+				log.Info("balance metrics shutting down")
+				return
+			}
+		}
+
+	}()
+}

op-service/metrics/balance_test.go

+package metrics
+
+import (
+	"math/big"
+	"testing"
+)
+
+type weiToEthTestCase struct {
+	input  *big.Int
+	output float64
+}
+
+func TestWeiToEther(t *testing.T) {
+	tests := []weiToEthTestCase{
+		{
+			input:  big.NewInt(1_000_000_000_000_000_000),
+			output: 1.0,
+		},
+		{
+			input:  big.NewInt(3_000_000_000_000_000_000),
+			output: 3.0,
+		},
+		{
+			input:  big.NewInt(3_456_789_000_000_000_000),
+			output: 3.456789,
+		},
+		{
+			input:  new(big.Int).Mul(big.NewInt(1_000_000), big.NewInt(1_000_000_000_000_000_000)),
+			output: 1_000_000,
+		},
+	}
+
+	for i, tc := range tests {
+		out := weiToEther(tc.input)
+		if out != tc.output {
+			t.Fatalf("test %v: expected %v but got %v", i, tc.output, out)
+		}
+	}
+
+}