metric: fix invalid type check for http.Flusher (#156)

This commit fixes an issue similar to d51a099.
The type check:
```
var x {
   ...
   flusher: w.(http.Flusher),
}
```
panics if `w` does not implement the `http.Flusher`
interface.

The correct way to check whether `w` implements
`http.Flusher` is:
```
var x {
   ...
}
if flusher, ok := w.(http.Flusher); ok {
   x.flusher = flusher
}
```

Signed-off-by: Andreas Auernhammer <[email protected]>

internal/metric/metric.go

@@ -131,13 +131,16 @@ func (m *Metrics) Count(h http.HandlerFunc) http.HandlerFunc {
 		m.requestActive.Inc()
 		defer m.requestActive.Dec()
 
-		h(&countResponseWriter{
+		var rw = countResponseWriter{
 			ResponseWriter: w,
-			flusher:        w.(http.Flusher),
 			succeeded:      m.requestSucceeded,
 			errored:        m.requestErrored,
 			failed:         m.requestFailed,
-		}, r)
+		}
+		if flusher, ok := w.(http.Flusher); ok {
+			rw.flusher = flusher
+		}
+		h(&rw, r)
 	}
 }
 
@@ -150,12 +153,15 @@ func (m *Metrics) Count(h http.HandlerFunc) http.HandlerFunc {
 // the application can handle.
 func (m *Metrics) Latency(h http.HandlerFunc) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
-		h(&latencyResponseWriter{
+		var rw = latencyResponseWriter{
 			ResponseWriter: w,
-			flusher:        w.(http.Flusher),
 			start:          time.Now(),
 			histogram:      m.requestLatency,
-		}, r)
+		}
+		if flusher, ok := w.(http.Flusher); ok {
+			rw.flusher = flusher
+		}
+		h(&rw, r)
 	}
 }