Fix prometheus extension due to renaming cse

[ritazh]

What this PR does / why we need it:
Update prometheus extension with new cse naming convention
Tested this extension on both master and agent

Which issue this PR fixes (optional, in fixes #<issue number>(, fixes #<issue_number>, ...) format, will close that issue when PR gets merged): fixes #
Fixes #2926

FYI @trstringer

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[trstringer]

Looks good to me! Thanks, @ritazh! 👍 😄

[CecileRobertMichon]

lgtm

[ghost]

@CecileRobertMichon @ritazh Do we think this should resolve the issues with extensions in VMSS as well? Or was there anything specific to VMSS still that will prevent extensions from working?

[CecileRobertMichon]

@neurot1cal I think VMSS might be a separate problem. I'm working on a PR to fix it.

[ritazh]

@neurot1cal The VMSS issue is a separate issue as described here: Azure/vm-scale-sets#26 We will keep you posted as soon as that fix is released by the VMSS team.

[CecileRobertMichon]

Yes @ritazh is correct there are two separate issues, the timeout one won't be addressed by my PR, only the naming incompatibility. PR opened at #3031.