indexer: fix simple semgrep issues (#2857)

Fix some very simple, low hanging fruit issues
that semgrep is finding in the indexer.

There are some more issues, but they have a larger
scope.
Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>

.changeset/gold-clocks-grin.md

+---
+'@eth-optimism/indexer': patch
+---
+
+Fix a couple semgrep issues

indexer/indexer.go

@@ -4,6 +4,7 @@ import (
 	"context"
 	"fmt"
 	"math/big"
+	"net"
 	"net/http"
 	"os"
 	"strconv"
@@ -234,7 +235,7 @@ func (b *Indexer) Serve() error {
 	middleware := server.LoggingMiddleware(b.metrics, log.New("service", "server"))
 
 	port := strconv.FormatUint(b.cfg.RESTPort, 10)
-	addr := fmt.Sprintf("%s:%s", b.cfg.RESTHostname, port)
+	addr := net.JoinHostPort(b.cfg.RESTHostname, port)
 
 	log.Info("indexer REST server listening on", "addr", addr)
 	return http.ListenAndServe(addr, middleware(c.Handler(b.router)))

indexer/metrics/metrics.go

 package metrics
 
 import (
-	"fmt"
+	"net"
 	"net/http"
 	"strconv"
 	"time"
@@ -218,7 +218,7 @@ func (m *Metrics) Serve(hostname string, port uint64) (*http.Server, error) {
 	mux := http.NewServeMux()
 	mux.Handle("/metrics", promhttp.Handler())
 	srv := new(http.Server)
-	srv.Addr = fmt.Sprintf("%s:%d", hostname, port)
+	srv.Addr = net.JoinHostPort(hostname, strconv.FormatUint(port, 10))
 	srv.Handler = mux
 	err := srv.ListenAndServe()
 	return srv, err