Add metrics.

[jmjoy]

Add metrics support.

[codecov-commenter]

Codecov Report

Merging #44 (5e87a20) into master (4d786ff) will decrease coverage by 5.41%.
The diff coverage is 1.69%.

@@            Coverage Diff             @@
##           master      #44      +/-   ##
==========================================
- Coverage   27.57%   22.16%   -5.42%     
==========================================
  Files          16       18       +2     
  Lines         475      591     +116     
==========================================
  Hits          131      131              
- Misses        344      460     +116     

Impacted Files	Coverage Δ
src/common/system_time.rs	0.00% <ø> (ø)
src/logging/logger.rs	0.00% <0.00%> (ø)
src/logging/record.rs	0.00% <ø> (ø)
src/metrics/meter.rs	0.00% <0.00%> (ø)
src/metrics/metricer.rs	0.00% <0.00%> (ø)
src/reporter/grpc.rs	0.00% <0.00%> (ø)
src/reporter/mod.rs	0.00% <ø> (ø)
src/reporter/print.rs	36.84% <28.57%> (-9.83%)	⬇️

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[wu-sheng]

You should provide a full list of the metric data types with explanations.

We have a short version on main repo docs, https://skywalking.apache.org/docs/main/latest/en/protocols/readme/#metrics which targets to proto file.

[jmjoy]

Updated.

[wu-sheng]

From a metrics APIs pespective, to ease the usage, the single value metric is usually separated as counter or gauge to provide different use patterns.

You could refer Java meter APIs from here, https://skywalking.apache.org/docs/skywalking-java/latest/en/setup/service-agent/java-agent/java-plugin-development-guide/#meter-plugin

[jmjoy]

I have refer to java agent and update the API.

[wu-sheng]

Just confirm, all metrics objects(counter, gauge, histogram), are they thread safe? Such as concurrency write, and write and report concurrently.

[jmjoy]

Just confirm, all metrics objects(counter, gauge, histogram), are they thread safe? Such as concurrency write, and write and report concurrently.

Yes, because they all implement Transform, which need Send and Sync, and the method increment of Counter, method add_value of Histogram, only need &self (the inner use atomic).

[wu-sheng]

LGTM