[Feature] Support gzip compression for http response (#4533)

After tablet level metrics is supported, the http metrics API may response
a very large body when a BE holds a large number of tablets, and cause heavy
network traffic.
This patch introduce http content compression to reduce network traffic.

be/src/http/http_request.cpp

@@ -122,9 +122,7 @@ const std::string& HttpRequest::param(const std::string& key) const {
 }
 
 void HttpRequest::add_output_header(const char* key, const char* value) {
-// #ifndef BE_TEST
     evhttp_add_header(evhttp_request_get_output_headers(_ev_req), key, value);
-// #endif
 }
 
 std::string HttpRequest::get_request_body() {