Add default settings to sample config of merge aggregator

[R290]

Use Case

My use case relies on a quick (order seconds) throughput from acquisition to writing to the database. The merge aggregator has a default window of 30 seconds and writes the metrics after this window has closed. I wanted to modify these settings and found this without any defaults listed: https://github.com/influxdata/telegraf/tree/master/config#aggregator-plugins. You actually need to look inside the code to find these:

telegraf/config/config.go

Lines 1360 to 1365 in 1cfc65a

	conf := &models.AggregatorConfig{
	Name: name,
	Delay: time.Millisecond * 100,
	Period: time.Second * 30,
	Grace: time.Second * 0,
	}

. The alternative is to enable the debug mode.

Expected behavior

The default values should be listed in the sample config of the merge aggregator plugin. Preferably with some description on what the Delay, Grace and Period settings actually do.

Actual behavior

The default 30 second merge window is sometimes too long and is not that straightforward to modify.

Additional info

Might apply to other aggregators as well?