Improve wording of labelmap action
Improve wording of labelmap action

Update the wording of the `labelmap` relabel action to make it more
clear that it acts on all the label names, rather than the list provided
by source_labels.

Signed-off-by: SuperQ <9bbf917dc63fbaa40c67921332d26cf16a50db75@gmail.com>

prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus