Logs entries in config.actionlog related to balancer round (

{what: "balancer.round"}

) contains a new section that contains details about balancer round timing:
{"times":

{ "selectionTimeMillis": <>, "throttleTimeMillis": <>, "migrationTimeMillis": <> }

Description of Linked Ticket

We observed that in large cluster with lots of collections/chunks the balancer can take several seconds to decide which chunks to move in each round.

The goal of this ticket is to include this time in the round details stored in config.actionlog