log: Store http request/responses in a log file (#4804)

When MINIO_TRACE_DIR is provided, create a new log file and store all
HTTP requests + responses data, body are excluded to reduce memory
consumption. MINIO_HTTP_TRACE=1 enables logging. Use non mem
consuming http req/resp recorders, the maximum is about 32k per request.
This logs to STDOUT, body logging is disabled for PutObject PutObjectPart
GetObject.

cmd/common-main.go

@@ -114,4 +114,6 @@ func handleCommonEnvVars() {
 		globalIsEnvBrowser = true
 		globalIsBrowserEnabled = bool(browserFlag)
 	}
+
+	globalHTTPTrace = os.Getenv("MINIO_HTTP_TRACE") != ""
 }