Data proxy: Log proxy errors using Grafana logger (#22174)

Now any errors logged by http.ReverseProxy are forwarded to 
Grafana's logger and includes more contextual information like
level (error), user id, org id, username, proxy path, referer and 
IP address.

Co-authored-by: Marcus Efraimsson <marcus.efraimsson@gmail.com>
(cherry picked from commit ff9556229a)

pkg/api/pluginproxy/ds_proxy.go

@@ -5,6 +5,7 @@ import (
 	"errors"
 	"fmt"
 	"io/ioutil"
+	"log"
 	"net"
 	"net/http"
 	"net/http/httputil"
@@ -17,7 +18,7 @@ import (
 	"golang.org/x/oauth2"
 
 	"github.com/grafana/grafana/pkg/bus"
-	"github.com/grafana/grafana/pkg/infra/log"
+	glog "github.com/grafana/grafana/pkg/infra/log"
 	"github.com/grafana/grafana/pkg/login/social"
 	m "github.com/grafana/grafana/pkg/models"
 	"github.com/grafana/grafana/pkg/plugins"
@@ -26,7 +27,7 @@ import (
 )
 
 var (
-	logger = log.New("data-proxy-log")
+	logger = glog.New("data-proxy-log")
 	client = newHTTPClient()
 )
 
@@ -57,6 +58,18 @@ type httpClient interface {
 	Do(req *http.Request) (*http.Response, error)
 }
 
+type logWrapper struct {
+	logger glog.Logger
+}
+
+// Write writes log messages as bytes from proxy
+func (lw *logWrapper) Write(p []byte) (n int, err error) {
+	withoutNewline := strings.TrimSuffix(string(p), "\n")
+	lw.logger.Error("Data proxy error", "error", withoutNewline)
+	return len(p), nil
+}
+
+// NewDataSourceProxy creates a new Datasource proxy
 func NewDataSourceProxy(ds *m.DataSource, plugin *plugins.DataSourcePlugin, ctx *m.ReqContext, proxyPath string, cfg *setting.Cfg) *DataSourceProxy {
 	targetURL, _ := url.Parse(ds.Url)
 
@@ -83,9 +96,12 @@ func (proxy *DataSourceProxy) HandleRequest() {
 		return
 	}
 
+	proxyErrorLogger := logger.New("userId", proxy.ctx.UserId, "orgId", proxy.ctx.OrgId, "uname", proxy.ctx.Login, "path", proxy.ctx.Req.URL.Path, "remote_addr", proxy.ctx.RemoteAddr(), "referer", proxy.ctx.Req.Referer())
+
 	reverseProxy := &httputil.ReverseProxy{
 		Director:      proxy.getDirector(),
 		FlushInterval: time.Millisecond * 200,
+		ErrorLog:      log.New(&logWrapper{logger: proxyErrorLogger}, "", 0),
 	}
 
 	transport, err := proxy.ds.GetHttpTransport()