Including `transaction` field in Dynamic Sampling Context must depend on transaction source

[vladanpaunovic]

In some SDKs (mainly JS Browser) we run into the issue of having "low quality" transaction names. These are transaction names that are basically useless to customers because they mess up grouping and by extension will mess up the sampling rules for dynamic sampling.

"Low quality transactions" are transactions that have a high cardinality in nature which boils down to URLs as transaction names (e.g. "example.com/resources/1398074123749127834/container/879872937819"). In order to filter these bad transactions names out, we only wanna include only low cardinality ones, i.e. parameterized or named in some other way (e.g. "example.com/resources/:resouceId/container/:containerId").

This all should be documented.