allow influx/prom outputs to have a custom prefix

[hellt]

Now, gnmic+$subscription_name parts are unconditionally prepended to a resulting metric which might be not what users want.
Processors can't help, since they are processed before the prefix added.

Proposal is to expose a prefix configuration option for prom/influx outputs which will allow to choose how prefix should look like. For example possible values:

metric-prefix: gnmic,$subcription-name # how it is now
metric-prefix: $string # only $string is appended
metric-prefix: gnmic # only gnmic is appended
metric-prefix: $subscription-name # only subscription-name is appended

[karimra]

yes, can be done

[hellt]

converted to FR #260