Correctly format the path to read data from npipe. (elastic#15997)

This is not a fix for elastic#15832 this will be tackle in another PR.
When http+npipe:// scheme were initially parsed by the url parser of
metric the generated PATH wasn't correctly setup up. So I've fixed the
parsing and the tests.

Now to test the behavior on Windows you will need a beat that expose metrics over
npipes, you can do so with a configuration like this.

```yaml
metricbeat.modules:
  - module: system

output.console: ~

http.enabled: true
http.host: \\.\pipe\custom
```yaml

After you can start a Metricbeat instance with the beat modules to
collect the metric information.

```yaml
metricbeat.modules:
- module: beat
  hosts: http+npipe://./pipe/custom
  metricsets:
  - stats
  - state
output.console: ~
```

If you start metricbeat with the `-e -d "*"` flags this will output any
metrics fetched from the running beat instance.

Fixes: elastic#15938
(cherry picked from commit 75ddc22)

metricbeat/mb/parse/url.go

@@ -212,7 +212,7 @@ func getURL(
 		u.Scheme = "http"
 		u.Host = "unix"
 	case "http+npipe":
-		p := strings.Replace(u.Path, "/pipe", `\\.pipe`, 1)
+		p := strings.Replace(u.Path, "/pipe", `\\.\pipe`, 1)
 		p = strings.Replace(p, "/", "\\", -1)
 		t = dialer.NewNpipeDialerBuilder(p)
 		u.Path = ""

metricbeat/mb/parse/url_test.go

@@ -97,7 +97,7 @@ func TestParseURL(t *testing.T) {
 		if assert.NoError(t, err) {
 			transport, ok := hostData.Transport.(*dialer.NpipeDialerBuilder)
 			assert.True(t, ok)
-			assert.Equal(t, `\\.pipe\custom`, transport.Path)
+			assert.Equal(t, `\\.\pipe\custom`, transport.Path)
 			assert.Equal(t, "http://npipe", hostData.URI)
 			assert.Equal(t, "http://npipe", hostData.SanitizedURI)
 			assert.Equal(t, "npipe", hostData.Host)
@@ -112,7 +112,7 @@ func TestParseURL(t *testing.T) {
 		if assert.NoError(t, err) {
 			transport, ok := hostData.Transport.(*dialer.NpipeDialerBuilder)
 			assert.True(t, ok)
-			assert.Equal(t, `\\.pipe\custom`, transport.Path)
+			assert.Equal(t, `\\.\pipe\custom`, transport.Path)
 			assert.Equal(t, "http://npipe/apath", hostData.URI)
 			assert.Equal(t, "http://npipe/apath", hostData.SanitizedURI)
 			assert.Equal(t, "npipe", hostData.Host)