Add watermill event histogram

[jhrozek]

We're able to get a histogram of how long does it take for the
individual watermill events to execute:

elapsed while executing the handler function in seconds
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.0005"}
0
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.001"}
0
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.0025"}
0
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.005"}
0
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.01"}
0
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.025"}
0
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.05"}
0
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.1"}
2
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.25"}
2
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="0.5"}
2
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="1"}
2
mediator_eventer_handler_execution_time_seconds_bucket{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true",le="+Inf"}
2
mediator_eventer_handler_execution_time_seconds_sum{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true"}
0.125726416
mediator_eventer_handler_execution_time_seconds_count{handler_name="github.com/stacklok/mediator/internal/engine.(*Executor).HandleEntityEvent-fm-internal.entity.event",success="true"}
2