Document all possible configurations allowed for MetricSink inputs and outputs

[mugugnu]

Currently documentation on PKS1.7 only reports 1 example for configuring MetricSink, where no inputs properties are defined. it's not clear if no input properties can be configured at all or if there are properties which can be specified to tweak telegraf behaviour. For example: would it be possibile to change the scraping interval ?
A complete list of properties which can be configured in MetricSink, LogSink,ClusterMetricSink,ClusterLogSink should be provided.

[cf-gitbot]

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