Update documentation to reference ContextSnapshotFactory

[philsttr]

The current example in the documentation uses ContextSnapshot.captureAllUsing(key -> true, registry), which was recently deprecated in micrometer-metrics/context-propagation#105

Please update the example to use non-deprecated methods, and describe usage of the recently introduced ContextSnapshotFactory in documentation where appropriate.

[marcingrzejszczak]

fixed via a5d5960