feat: external calculation via gRPC servers

[gauron99]

STATUS

ready to be worked on prerequisite PR merged.

this PR is a follow up for this. Both of these are part of the Parent PR that Ive separated into 2 for easier and more clear reviews etc.

gRPC servers summary

this PR implements new way to manipulate/alter/ external metrics.
Each server has:

• its own url to connect to where gRPC server is running with pre-defined .proto structure (KEDA is gRPC client that will try to connect)
• optional security certificate directory (user gives certificate directory - similar to metrics server certificate)
• needs to have unique name (this is mandatory, last grpc server can be used in formula for further manipulation same as a trigger)
• timeout to initially connect

explanation

proto file exists in pkg/externalscaling/api which describes the structure of following generated KEDA client side for gRPC and is the base for the gRPC server that user can implement.
If this server is implemented, structure in SO is defined and KEDA successfully connects - this feature implements possibility for user to use defined external triggers, send them to this server where they can manipulate them however they want and return whatever they want in the same structure. This structure is converted to the one KEDA uses ([]external_metrics.ExternalMetricValue) and simply returns the metrics, passing it to HPA. (possibly through formula and then passing it to HPA if defined)

possible struct view in SO

struct name is to be determined

advanced:
 modifiers:
  target: "2"
  formula: "xxx"
  external:
  - name: grpc_one
    url: localhost:7999
    timeout: 10
  - name: grpc_two
    url: localhost:8000
    timeout: 1m12s

Checklist

• 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 the documentation on (repo)
• Commits are signed with Developer Certificate of Origin (DCO - learn more)

Closes #4583

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[tomkerkhove]

Any update on this? If we don't hear back in the near future we'd likely need to close this PR

[gauron99]

I have rebased part of what needs to be rebased. its not complete but more pressing issues were at hand. I was told its not exactly a wanted issue but would be nice to have ready. If youd like this to be implemented I can up the priority.
Or if not at all feel free to close it

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed in 7 days if no further activity occurs. Thank you for your contributions.

[stale]

This issue has been automatically closed due to inactivity.