Added datadogMetricsService and fix datadogNamespace key in triggerAu…

[alvarobacelar]

Included datadogMetricsService and fixed datadogNamespace key in triggerAuthenticatio specs. When I have been Tried this configurations I got errors. After I fix this keys, provide in this PR, works well.

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Fixes #

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[zroubalik]

Thanks, duplicate of: #1451

[alvarobacelar]

Hello @zroubalik , but this PR fix the namespace as well.

https://github.com/kedacore/keda-docs/pull/1469/files#diff-015e70f7405a2745734c1d02b964c2a3990ff85783e2bbf9f31a12037c115552R144

[zroubalik]

@alvarobacelar thanks, haven't notice that. Could you please rebase this PR ?

[alvarobacelar]

@zroubalik I've been done.

[alvarobacelar]

Hi @zroubalik. is it everything ok with this PR?