save span with minmum latency us

src/brpc/span.cpp

@@ -59,6 +59,9 @@ BRPC_VALIDATE_GFLAG(rpcz_keep_span_seconds, PositiveInteger);
 
 DEFINE_bool(rpcz_keep_span_db, false, "Don't remove DB of rpcz at program's exit");
 
+DEFINE_int64(rpcz_save_span_min_latency_us, 0, "The minimum latency microseconds of span saved");
+BRPC_VALIDATE_GFLAG(rpcz_save_span_min_latency_us, NonNegativeInteger);
+
 struct IdGen {
     bool init;
     uint16_t seq;
@@ -574,6 +577,11 @@ leveldb::Status SpanDB::Index(const Span* span, std::string* value_buf) {
     // of time window.
 
     const int64_t start_time = span->GetStartRealTimeUs();
+    const int64_t latency_us = span->GetEndRealTimeUs() - start_time;
+    // if latency_us < FLAGS_rpcz_save_span_min_latency_us, don't save this span
+    if (latency_us < FLAGS_rpcz_save_span_min_latency_us) {
+        return leveldb::Status::OK();
+    }
     BriefSpan brief;
     brief.set_trace_id(span->trace_id());
     brief.set_span_id(span->span_id());
@@ -583,7 +591,7 @@ leveldb::Status SpanDB::Index(const Span* span, std::string* value_buf) {
     brief.set_request_size(span->request_size());
     brief.set_response_size(span->response_size());
     brief.set_start_real_us(start_time);
-    brief.set_latency_us(span->GetEndRealTimeUs() - start_time);
+    brief.set_latency_us(latency_us);
     brief.set_full_method_name(span->full_method_name());
     if (!brief.SerializeToString(value_buf)) {
         return leveldb::Status::InvalidArgument(