refactor gcp stackdriver scaler

[omerap12]

Provide a description of what has been changed

Checklist

• When introducing a new scaler, I agree with the scaling governance policy
• I have verified that my change is according to the deprecations & breaking changes policy
• Tests have been added
• Changelog has been updated and is aligned with our changelog requirements
• A PR is opened to update our Helm chart (repo) (if applicable, ie. when deployment manifests are modified)
• A PR is opened to update the documentation on (repo) (if applicable)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Fixes #

Relates to #5797

[zroubalik]

/run-e2e gcp
Update: You can check the progress here

[wozniakjan]

lgtm, thank you!

minor nits below

[wozniakjan]

just noticed the imports are not sorted the way linter is configured, can you please run following command? (or address the imports sort manually :))

gci write . --skip-generated -s standard -s default -s prefix(github.com/kedacore/keda)

I will retrigger the e2e tests after that

[omerap12]

just noticed the imports are not sorted the way linter is configured, can you please run following command? (or address the imports sort manually :))

gci write . --skip-generated -s standard -s default -s prefix(github.com/kedacore/keda)

I will retrigger the e2e tests after that

done