Performance Improvement: EaseAgent has a relatively large impact on cpu and latency

[Oseenix]

Is your feature request related to a problem? Please describe.
EaseAgent has a relatively large impact on cpu and latency

CPU usage is related to Span number in a transaction: from about 3.6% of 1 span to 12.2% of 9 spans, which is too hight.
Throughput has dropped from about 1800 to 1500 nearly.

This benchmark report can be found in EaseAgent/doc/benchmark2.md.

Describe the solution you'd like

CPU profile: vets-700.html.txt, this file can be downloaded and the suffix can be changed to an html type file.

After review the profile file, maybe we could improve by:

• don't call calculate sizeInBytes() before encode, which requiring a new async module;

• in brave lib, before report, the inner real Span(MutableSpan) instance will be converted to ReportSpan(zipkin2.Span/com.megaease...tracing.ReportSpan), try do not do this conversion every time.

• try use the lock-free disruptor for async reporter.

• try find other hight performance json encoder lib.

Welcome to review the benchmark report and contribute your idea of improvement.