add avg latency for prometheus metrics

[Huixxi]

What problem does this PR solve?

Issue Number:

Problem Summary:

What is changed and the side effects?

Changed:

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• Performance effects(性能影响):

• Breaking backward compatibility(向后兼容性):

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[Huixxi]

Fix #2012

[zyearn]

这个可能得加在其它地方，这个函数export的是Histogram
类型 https://prometheus.io/docs/concepts/metric_types/#histogram

[wwbmmm]

这个可能得加在其它地方，这个函数export的是Histogram 类型 https://prometheus.io/docs/concepts/metric_types/#histogram

应该不是Histogram，是Summary类型
https://github.com/apache/incubator-brpc/blob/master/src/brpc/builtin/prometheus_metrics_service.cpp#L159

[zyearn]

这个可能得加在其它地方，这个函数export的是Histogram 类型 https://prometheus.io/docs/concepts/metric_types/#histogram

应该不是Histogram，是Summary类型 https://github.com/apache/incubator-brpc/blob/master/src/brpc/builtin/prometheus_metrics_service.cpp#L159

嗯嗯是Summary。

[serverglen]

LGTM

[lorinlee]

summary的quantile写avg是Prometheus支持的吗，这个有相关文档不，我搜了下只找到了quantile里是百分比数字的，还有_sum和_count

[Huixxi]

label可以是自定义字符串，这里命名是“avg”，具体的值是si->latency_avg。