Support metrics generation

[spencergilbert]

We should add the ability for lading to produce metric style events to unblock additional soak test configurations. Depending on the metrics and "flavor" of metrics this may be a new generator or a new variant of existing generators.

We'll likely want to create separate tickets for the metric formats and flavors we want to include in lading

[blt]

Seems straightforward to do as a payload, defined here. Then we'd just need to expand the payloads supported by whatever generator. Possibly relates to #55.