This patch adds a metrics API implementation.
The API supports get and set the metric relabel config.
Seastar supports metrics relabeling in runtime, following Prometheus
relabel_config.
Based on metrics and label name, a user can add or remove labels,
disable a metric and set the skip_when_empty flag.
The metrics-config API support such configuration to be done using the
RestFull API.
As it's a new API it is placed under the V2 path.
After this patch the following API will be available
'http://localhost:10000/v2/metrics-config/' GET/POST.
For example:
To get the current config:
```
curl -X GET --header 'Accept: application/json' 'http://localhost:10000/v2/metrics-config/'
```
To set a config:
```
curl -X POST --header 'Content-Type: application/json' --header 'Accept: application/json' -d '[ \
{ \
"source_labels": [ \
"__name__" \
], \
"action": "replace", \
"target_label": "level", \
"replacement": "1", \
"regex": "io_que.*" \
} \
]' 'http://localhost:10000/v2/metrics-config/'
```
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