Add a actuator metrics API

[seeflood]

What would you like to be added:
Layotto actuator API is designed as a collection of features to help users monitor and manage Layotto and applications during runtime,like health checking,metadata query and metrics query.
However,there is already a metrics api in MOSN,which means Layotto users have to configurate TWO http api in the configuration file: one for Layotto actuator API and one for MOSN metrics API.
We want to make it simpler for users. By adding a Layotto actuator metrics API,users can query metrics data including MOSN metrics and Layotto metrics.

Tasks included:

• add a Layotto actuator metrics API
• Layotto actuator metrics API can query MOSN metrics data

chinese:
任务：设计实现actuator metrics API，用于统计、展示metrics指标
背景：
Layotto Actuator API被设计用来提供一系列功能、帮助用户在运行时监控、管理应用，包括健康检查，元数据查询，metrics数据查询等功能。
然而，MOSN里也有个metrics API，这就意味着用户得配两个http端口，一个用来跑Layotto actuator API，一个用于MOSN metrics API。不仅配置麻烦，运维、使用API也麻烦。
我们希望让用户用起来更简单，只需要开一个端口。我们可以加个actuator metrics API，这个API既能查到Layotto metrics数据又能查到MOSN metrics数据。

要做的事情包括：

• 加个Layotto Actuator metrics API
• 这个API能查MOSN metrics数据

参考资料：
MOSN metrics

Why is this needed:
Make it simpler for users to configurate and monitor Layotto during runtime.

[wangyanghack]

hi, I want to have a try. Please assign me, thanks very much.

[seeflood]

@wangyanghack Cool !
Since this is a relatively common feature, we recommend that you briefly write the design before coding, such as what the api path is, what is the returned data structure, and how to implement it. So as to avoid rework.
Welcome to the Layotto community!

鉴于这是一个比较通用的功能，我们建议在编码之前先简单写一下设计、发出来大家讨论下，比如api的路径是啥、返回的数据结构是啥，大概如何实现等等。这样避免写完了代码后返工哈

[wangyanghack]

Yes, it is necessary to design and have a talk before coding.

[seeflood]

@wangyanghack Hi ,are u still working on it?