[SCB-6] add metrics samples readme

[zhengyangyong]

Signed-off-by: zhengyangyong yangyong.zheng@huawei.com

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Add README.md for metrics health check sample and write metrics to file sample

[coveralls]

Coverage increased (+0.006%) to 87.189% when pulling c44586f on zhengyangyong:SCB-6-SampleDoc into e1137b4 on apache:master.

[asifdxtreme]

LGTM