document the metric_set_limit option

[jackshirazi]

What does this PR do?

document the metric_set_limit option per comments in #2148 - making the option not internal and expand the documentation

[ghost]

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• Start Time: 2022-07-21T11:02:36.441+0000

• Duration: 48 min 54 sec

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[eyalkoren]

1. Transaction names is only one specific case of what's affecting the number of metricsets. The general factor that determine metricsets cardinality is the metrics labels (see MetricRegistryTest). So I propose talking about labels, but saying that it's better to reduce cardinality rather than increasing the limit is good, in which case you can mention transaction name as one option
2. You need to run ConfigurationExporterTest for this to be actually added to the documentation

[jackshirazi]

Thanks @eyalkoren , is the latest version good enough, or still further changes needed?