Logs PushRequest data.

This will allows to find information about received size and total entries per tenant.

Example of a log from my dev testing:

```
level=debug ts=2021-01-15T11:16:21.735663076Z caller=http.go:67 org_id=3927 traceID=11c4774c6ec4bbf4 msg="push request parsed" path=/loki/api/v1/push content-type=application/x-protobuf body-size="11 kB" streams=5 entries=298 streamLabelsSize="1.9 kB" entriesSize="45 kB" totalSize="47 kB"
```

Of course this means we can use LogQL on this.

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

pkg/distributor/http.go

@@ -4,6 +4,8 @@ import (
 	"math"
 	"net/http"
 
+	"github.com/dustin/go-humanize"
+	"github.com/go-kit/kit/log/level"
 	"github.com/weaveworks/common/httpgrpc"
 
 	"github.com/cortexproject/cortex/pkg/util"
@@ -12,6 +14,7 @@ import (
 	"github.com/grafana/loki/pkg/logproto"
 	"github.com/grafana/loki/pkg/logql/unmarshal"
 	unmarshal_legacy "github.com/grafana/loki/pkg/logql/unmarshal/legacy"
+	lokiutil "github.com/grafana/loki/pkg/util"
 )
 
 var contentType = http.CanonicalHeaderKey("Content-Type")
@@ -20,7 +23,6 @@ const applicationJSON = "application/json"
 
 // PushHandler reads a snappy-compressed proto from the HTTP body.
 func (d *Distributor) PushHandler(w http.ResponseWriter, r *http.Request) {
-
 	req, err := ParseRequest(r)
 	if err != nil {
 		http.Error(w, err.Error(), http.StatusBadRequest)
@@ -44,22 +46,53 @@ func (d *Distributor) PushHandler(w http.ResponseWriter, r *http.Request) {
 func ParseRequest(r *http.Request) (*logproto.PushRequest, error) {
 	var req logproto.PushRequest
 
-	switch r.Header.Get(contentType) {
+	logger := util.WithContext(r.Context(), util.Logger)
+	body := lokiutil.NewSizeReader(r.Body)
+	contentType := r.Header.Get(contentType)
+
+	defer func() {
+		var (
+			entriesSize      int64
+			streamLabelsSize int64
+			totalEntries     int64
+		)
+
+		for _, s := range req.Streams {
+			streamLabelsSize += int64(len(s.Labels))
+			for _, e := range s.Entries {
+				totalEntries++
+				entriesSize += int64(len(e.Line))
+			}
+		}
+		level.Debug(logger).Log(
+			"msg", "push request parsed",
+			"path", r.URL.Path,
+			"content-type", contentType,
+			"body-size", humanize.Bytes(uint64(body.Size())),
+			"streams", len(req.Streams),
+			"entries", totalEntries,
+			"streamLabelsSize", humanize.Bytes(uint64(streamLabelsSize)),
+			"entriesSize", humanize.Bytes(uint64(entriesSize)),
+			"totalSize", humanize.Bytes(uint64(entriesSize+streamLabelsSize)),
+		)
+	}()
+
+	switch contentType {
 	case applicationJSON:
 		var err error
 
 		if loghttp.GetVersion(r.RequestURI) == loghttp.VersionV1 {
-			err = unmarshal.DecodePushRequest(r.Body, &req)
+			err = unmarshal.DecodePushRequest(body, &req)
 		} else {
-			err = unmarshal_legacy.DecodePushRequest(r.Body, &req)
+			err = unmarshal_legacy.DecodePushRequest(body, &req)
 		}
 
 		if err != nil {
 			return nil, err
 		}
 
 	default:
-		if err := util.ParseProtoReader(r.Context(), r.Body, int(r.ContentLength), math.MaxInt32, &req, util.RawSnappy); err != nil {
+		if err := util.ParseProtoReader(r.Context(), body, int(r.ContentLength), math.MaxInt32, &req, util.RawSnappy); err != nil {
 			return nil, err
 		}
 	}

pkg/util/reader.go

@@ -0,0 +1,31 @@
+package util
+
+import (
+	"io"
+)
+
+type sizeReader struct {
+	size int64
+	r    io.Reader
+}
+
+type SizeReader interface {
+	io.Reader
+	Size() int64
+}
+
+// NewSizeReader returns an io.Reader that will have the number of bytes
+// read from r available.
+func NewSizeReader(r io.Reader) SizeReader {
+	return &sizeReader{r: r}
+}
+
+func (v *sizeReader) Read(p []byte) (int, error) {
+	n, err := v.r.Read(p)
+	v.size += int64(n)
+	return n, err
+}
+
+func (v *sizeReader) Size() int64 {
+	return v.size
+}