Add Splunk_hec label to metrics output

[FluffySocks32]

Hi,

When using the splunk_hec output the label fails to be added, resulting in the field showing up as null from the statsd output.

I added the following below line 107 of the splunk_hec template, and it successfully adds the label back in.

metrics_mapping: | #!blobl meta label = $label | ""

Long story short, I can't add this in as a PR. Would it be possible to confirm this is actually a fix?

Thank you