Split "collect" and "export" functionalities into separate subcommands.

[mkalcok]

This is a workaround for a memory leak that occurred when the long running process (exporter) was using libjuju directly. Things that have been changed:

• CLI was split into two (mandatory) sub-commands export and collect
• prometheus-juju-exporter collect gathers the data from controller and prints the output to STDOUT as a json
• prometheus-juju-exporter export runs the Prometheus exporter and periodically executes prometheus-juju-exporter collect as a subprocess to get updated data

With this structure, libjuju objects are created only in the short-lived process prometheus-juju-exporter collect and therfore can't cause memory leak in the long living process prometheus-juju-exporter export

Note: This is just a working version to get some feedback. Unit tests were not adjusted yet.

TODO before potential merge:

• Update UTs
• Update and test snap version
• Update charm that uses this exporter

[esunar]

I've checked the memory consumption with 10, 100 executions. It seems stable.

[mkalcok]

superseded with #14 which is a correct fix for the memory issue.