[Fix #306] Add de_slash with de_slash_separator feature for Datadog Logs support

[rymir]

This pull request mirrors the de_dot feature, adding an additional de_slash functionality, so that Datadog configuration can be supported when using recommended kubernetes labels https://kubernetes.io/docs/concepts/overview/working-with-objects/common-labels/

[jcantrill]

released https://rubygems.org/gems/fluent-plugin-kubernetes_metadata_filter/versions/2.9.0