Add holt_winters backwards compatibility in Cortex parser

[afhassan]

What this PR does:
Cortex supports holt_winters() for backwards compatibility. However this behaviour is currently broken as the function is only added to the engine parser at runtime and is not added to cortex_parser when it is initialized, causing queries to fail with error:

bad_data: 1:1: parse error: unknown function with name "holt_winters"

Which issue(s) this PR fixes:
Fixes #

Checklist

• Tests updated
• Documentation added
• CHANGELOG.md updated - the order of entries should be [CHANGE], [FEATURE], [ENHANCEMENT], [BUGFIX]

[yeya24]

Can we also refactor the code a little bit? The same code exists in

cortex/pkg/querier/querier.go

Line 723 in caedb08

func EnableExperimentalPromQLFunctions(enablePromQLExperimentalFunctions, enableHoltWinters bool) {

.

We should try to consolidate the code. It sounds a miss of not adding holt winters here when we added the cortex parser module. If that's the case we should remove the same code above.

[yeya24]

Can you add a test?
Also I am trying to understand why our integration test didn't capture this issue before. https://github.com/cortexproject/cortex/blob/master/integration/backward_compatibility_test.go#L161

[yeya24]

Also I think this should be behind a feature flag as well. Same as

cortex/pkg/querier/querier.go

Line 723 in caedb08

func EnableExperimentalPromQLFunctions(enablePromQLExperimentalFunctions, enableHoltWinters bool) {

. Even though we hardcode to true today we may want to expose a flag so that we can disable the fallback in the future.

[afhassan]

Refactored it to register holt_winters for both parsers in the same place with a flag set to true.

The reason backward_compatibility_test did not catch the issue is that cortexparser was never used in it. We use it for validation in query-frontend, but there were no flags set that use cortexparser for validation. I added max_query_limit and that triggered the error.