Support metrics publishing for hooks

[diafour]

The idea is that the hook can return a set of JSON objects and the Shell-operator will publish them as metrics.

For example:

Hook writes JSON objects to a temporary file $METRICS_PATH

{"name":"metric_gauge", "set":34.12, "labels":{"label1":"val1", ... }}
{"name":"metric_count", "add":1, "labels":{"label1":"val1", ... }}

Shell-operator parse these objects and publish them via /metrics endpoint:

metric_gauge{"hook":"hook.sh", "label1":"val1"} 34.12
metric_count{"hook":"hook.sh", "label1":"val1"} 1

Note 1: "add" operation is also available for the gauge metric.

Note 2: there are no checks for the uniqueness of the "name" value. Metric name is not prefixed with "shell_operator_".

Note 3: a developer should check, that similar metrics have a similar set of labels or an error will occur and hook will restart.

Note 4: hook label is added automatically.

[diafour]

done in #165