Add expvars to the scheduler

[zippolyte]

What does this PR do?

Add expvars to the scheduler:

• number of queues and their interval and size
• number of checks entered

We get something like this:
"scheduler": {"ChecksEntered": 3, "Queues": [{"Interval":15,"Size":3}], "QueuesCount": 1}

[hush-hush]

please also update the pkg/collector/dist/conf.d/go_expvar.yaml.default to add the new variable to the check.