Allow access to stdout/stderr of Prometheus/PushGateway/etc

[hatchan]

In #54 we disabled the output of Prometheus and Pushgateway. If Prometheus/pushgateway doesn't start correctly then we only show that it didn't work correctly. But we should also allow access to the logs.

Issue #6 describes a few ways:

We should store this data into a buffer (maybe only collecting the last x bytes) and then expose this to the user.

The main way of retrieving this information would be through the am's api. The explorer can display this information. We can even create a live view of these logs through web-sockets or server side events.

Another way of using this data would be if an error occurred, that am would dump these logs into a file. These can then be used by the user when creating a bug report.

Finally a cli argument can be added that will make the Prometheus stdout/stderr go to into am's stdout/stderr.