feat: add support for metricLabelSelector with matchLabels

[lelithium]

When scaling an HPA with a Custom Metric, it is possible to specify a selector to specify which metric to target in the API. See https://kubernetes.io/docs/reference/kubernetes-api/workload-resources/horizontal-pod-autoscaler-v2/ section metrics.object.metric.selector
As a (truncated) example:

apiVersion: autoscaling/v2beta2
kind: HorizontalPodAutoscaler
...
spec:
...
  metrics:
  - type: Object
    object:
      metric:
        name: my_custom_metric
        selector: 
          matchLabels: 
            my_label: "my_value"
      describedObject:
        apiVersion: "/v1"
        kind: "Service"
        name: "my_service"
...

Will yield API calls to /apis/custom.metrics.k8s.io/v1beta1/namespaces/$NAMESPACE/services/my_service/my_custom_metric?metricLabelSelector=my_label%3Dmy_value

This PR adds support for such selectors.

[machine424]

Hello @lelithium

I knew someone would ask for this someday.

I'm no longer able to do manual testing before releasing anymore, I'll be adding some tests (maybe some e2e).

I'll ping you when it's done, you can then rebase and add yours.

[machine424]

I recommend we use the built-in urllib library for this

>>> import urllib.parse
>>> urllib.parse.urlencode({"foo": "bar", "bar": "foo"})
'foo=bar&bar=foo'

(no need to do all of this ourselves)

[lelithium]

Good catch ! Will fix in next commit

[lelithium]

Hey @machine424 , thanks for the update here !

I understand the need for some kind of testing. FYI, this particular version of the code I've been using in a staging environment for about a week, and I haven't encountered any issues with it

[machine424]

Hello @lelithium

I've just merged 8bddc0d adding the tests and their utils.