move common metrics-related funcs to internal package

Signed-off-by: Dave Henderson <dhenderson@gmail.com>

internal/metrics/metrics.go

@@ -0,0 +1,39 @@
+package metrics
+
+import (
+	"net/http"
+	"strconv"
+)
+
+func SanitizeCode(s int) string {
+	switch s {
+	case 0, 200:
+		return "200"
+	default:
+		return strconv.Itoa(s)
+	}
+}
+
+// Only support the list of "regular" HTTP methods, see
+// https://developer.mozilla.org/en-US/docs/Web/HTTP/Methods
+var methodMap = map[string]string{
+	"GET": http.MethodGet, "get": http.MethodGet,
+	"HEAD": http.MethodHead, "head": http.MethodHead,
+	"PUT": http.MethodPut, "put": http.MethodPut,
+	"POST": http.MethodPost, "post": http.MethodPost,
+	"DELETE": http.MethodDelete, "delete": http.MethodDelete,
+	"CONNECT": http.MethodConnect, "connect": http.MethodConnect,
+	"OPTIONS": http.MethodOptions, "options": http.MethodOptions,
+	"TRACE": http.MethodTrace, "trace": http.MethodTrace,
+	"PATCH": http.MethodPatch, "patch": http.MethodPatch,
+}
+
+// SanitizeMethod sanitizes the method for use as a metric label. This helps
+// prevent high cardinality on the method label. The name is always upper case.
+func SanitizeMethod(m string) string {
+	if m, ok := methodMap[m]; ok {
+		return m
+	}
+
+	return "OTHER"
+}

internal/metrics/metrics_test.go

@@ -0,0 +1,28 @@
+package metrics
+
+import (
+	"strings"
+	"testing"
+)
+
+func TestSanitizeMethod(t *testing.T) {
+	tests := []struct {
+		method   string
+		expected string
+	}{
+		{method: "get", expected: "GET"},
+		{method: "POST", expected: "POST"},
+		{method: "OPTIONS", expected: "OPTIONS"},
+		{method: "connect", expected: "CONNECT"},
+		{method: "trace", expected: "TRACE"},
+		{method: "UNKNOWN", expected: "OTHER"},
+		{method: strings.Repeat("ohno", 9999), expected: "OTHER"},
+	}
+
+	for _, d := range tests {
+		actual := SanitizeMethod(d.method)
+		if actual != d.expected {
+			t.Errorf("Not same: expected %#v, but got %#v", d.expected, actual)
+		}
+	}
+}