lib/filestream: do not measure read / write duration from / to in-memory buffers

Measuring read / write duration from / to in-memory buffers has little sense,
since it will be always fast. It is better to measure read / write duration from / to
real files at vm_filestream_write_duration_seconds_total and vm_filestream_read_duration_seconds_total metrics.
This also reduces overhead on time.Now() and Histogram.UpdateDuration() calls
per each filestream.Reader.Read() and filestream.Writer.Write() call when the data is read / written from / to in-memory buffers.

This is a follow-up for 2f63dec2e3

lib/filestream/filestream.go

@@ -133,11 +133,6 @@ var (
 
 // Read reads file contents to p.
 func (r *Reader) Read(p []byte) (int, error) {
-	startTime := time.Now()
-	defer func() {
-		d := time.Since(startTime).Seconds()
-		readDuration.Add(d)
-	}()
 	readCallsBuffered.Inc()
 	n, err := r.br.Read(p)
 	readBytesBuffered.Add(n)
@@ -155,8 +150,11 @@ type statReader struct {
 }
 
 func (sr *statReader) Read(p []byte) (int, error) {
+	startTime := time.Now()
 	readCallsReal.Inc()
 	n, err := sr.File.Read(p)
+	d := time.Since(startTime).Seconds()
+	readDuration.Add(d)
 	readBytesReal.Add(n)
 	return n, err
 }
@@ -268,11 +266,6 @@ var (
 
 // Write writes p to the underlying file.
 func (w *Writer) Write(p []byte) (int, error) {
-	startTime := time.Now()
-	defer func() {
-		d := time.Since(startTime).Seconds()
-		writeDuration.Add(d)
-	}()
 	writeCallsBuffered.Inc()
 	n, err := w.bw.Write(p)
 	writtenBytesBuffered.Add(n)
@@ -309,8 +302,11 @@ type statWriter struct {
 }
 
 func (sw *statWriter) Write(p []byte) (int, error) {
+	startTime := time.Now()
 	writeCallsReal.Inc()
 	n, err := sw.File.Write(p)
+	d := time.Since(startTime).Seconds()
+	writeDuration.Add(d)
 	writtenBytesReal.Add(n)
 	return n, err
 }