caddyhttp: performance improvement in HeaderRE Matcher (#4143)

Below is the report using `benchstat` and cmd:

`go test -run=BenchmarkHeaderREMatcher -bench=BenchmarkHeaderREMatcher -benchmem -count=10`

```
name                old time/op    new time/op    delta
HeaderREMatcher-16     869ns ± 1%     658ns ± 0%  -24.29%  (p=0.000 n=10+10)

name                old alloc/op   new alloc/op   delta
HeaderREMatcher-16      144B ± 0%      112B ± 0%  -22.22%  (p=0.000 n=10+10)

name                old allocs/op  new allocs/op  delta
HeaderREMatcher-16      7.00 ± 0%      5.00 ± 0%  -28.57%  (p=0.000 n=10+10)
```

modules/caddyhttp/matchers.go

@@ -24,6 +24,7 @@ import (
 	"path/filepath"
 	"regexp"
 	"sort"
+	"strconv"
 	"strings"
 
 	"github.com/caddyserver/caddy/v2"
@@ -935,14 +936,14 @@ func (mre *MatchRegexp) Match(input string, repl *caddy.Replacer) bool {
 
 	// save all capture groups, first by index
 	for i, match := range matches {
-		key := fmt.Sprintf("%s.%d", mre.phPrefix, i)
+		key := mre.phPrefix + "." + strconv.Itoa(i)
 		repl.Set(key, match)
 	}
 
 	// then by name
 	for i, name := range mre.compiled.SubexpNames() {
 		if i != 0 && name != "" {
-			key := fmt.Sprintf("%s.%s", mre.phPrefix, name)
+			key := mre.phPrefix + "." + name
 			repl.Set(key, matches[i])
 		}
 	}

modules/caddyhttp/matchers_test.go

@@ -692,6 +692,32 @@ func TestHeaderREMatcher(t *testing.T) {
 	}
 }
 
+func BenchmarkHeaderREMatcher(b *testing.B) {
+
+	i := 0
+	match := MatchHeaderRE{"Field": &MatchRegexp{Pattern: "^foo(.*)$", Name: "name"}}
+	input := http.Header{"Field": []string{"foobar"}}
+	var host string
+	err := match.Provision(caddy.Context{})
+	if err != nil {
+		b.Errorf("Test %d %v: Provisioning: %v", i, match, err)
+	}
+	err = match.Validate()
+	if err != nil {
+		b.Errorf("Test %d %v: Validating: %v", i, match, err)
+	}
+
+	// set up the fake request and its Replacer
+	req := &http.Request{Header: input, URL: new(url.URL), Host: host}
+	repl := caddy.NewReplacer()
+	ctx := context.WithValue(req.Context(), caddy.ReplacerCtxKey, repl)
+	req = req.WithContext(ctx)
+	addHTTPVarsToReplacer(repl, req, httptest.NewRecorder())
+	for run := 0; run < b.N; run++ {
+		match.Match(req)
+	}
+}
+
 func TestVarREMatcher(t *testing.T) {
 	for i, tc := range []struct {
 		desc       string