dripline reporting of run metrics

[laroque]

It would be great if we had an online way of distributing metrics related to a run at runtime (ie without needing further data processing). The suggested solution is two parts:

1. Modify message_relayer, adding a method for sending a data log broadcast message. This is then available to any nodes that have useful metrics which we want to log... should be a relatively simple extension of the existing slack message methods.

2. Modify the event_builder to count triggers:

• upon receiving a midge:s_start, reset the internal trigger counter to 0
• upon receiving a midge:s_stop, use the above to broadcast the current trigger count
• whenever the internal state is changed to triggered, also ++ the trigger counter
• add whatever configuration is required for using the above broadcast: want options for disabling and probably an option for specifying the routing key target used.