doc(katib): update push-based metrics collector.

[Electronic-Waste]

This PR aims at guiding users using push-based metrics collection: kubeflow/katib#2340.

I rewrote the metrics-collector chapter and divided it into three parts:

1. Overview: Introduce the type and the config of the metrics collector.
2. Pull-based Metrics Collector: The original part of this chapter. I removed None MC.
3. Push-based Metrics Collector: Describe the usage of Push MC, with an example attached.

cc👀 @kubeflow/wg-automl-leads . Please let me know if you have any suggestions.

[andreyvelich]

Thanks for adding this @Electronic-Waste!

/assign @hbelmiro @StefanoFioravanzo

[Electronic-Waste]

I left some explanations for the fields in tune function. If you have any questions, don't hesitate to contact me.

[hbelmiro]

/lgtm

[andreyvelich]

Thank you for this. I left a few comments.
/assign @kubeflow/wg-training-leads

[andreyvelich]

Let's use simple example here and remove all unnecessary parameters and function calls.

[Electronic-Waste]

@andreyvelich Sorry, I may not fully understand what you mean "unnecessary parameters and function calls".

I copied the example in the get-started chapter and replaced print with report_metrics(). Do you mean that I don't need to call the tune() function and just define a main function in the example?

[andreyvelich]

I think, we should just show example how user can use tune API with push-based metrics collector without any additional parameters they should set. For example:

import kubeflow.katib as katib

def objective(parameters):
  import time
  import kubeflow.katib as katib
  time.sleep(5)
  result = 4 * int(parameters["a"])
  # Push metrics to Katib DB.
  katib.report_metrics({"result": result})

katib.KatibClient().tune(
  name="push-metrics-exp",
  objective=objective,
  parameters= {"a": katib.search.int(min=10, max=20)}
  objective_metric_name="result",
  max_trial_count=2,
  metrics_collector_config={"kind": "Push"},
  # When SDK is released, replace it with packages_to_install=["kubeflow-katib==0.18.0"] 
  packages_to_install=["git+https://github.com/kubeflow/katib.git@master#subdirectory=sdk/python/v1beta1"],
)

That should allow user to focus on important changes they need to make to try this out.

[Electronic-Waste]

@andreyvelich Thanks for your clarification! I'll update the blog.

[andreyvelich]

You might need to explain how report_metrics() functions works separately. E.g. user needs to install kubeflow-katib SDK in their environment, and the report_metrics() will automatically add the timestamp for metrics.

[Electronic-Waste]

SGTM. I'll add the explanation to report_metrics().

[Electronic-Waste]

@kubeflow/wg-automl-leads I've made some changes to the PR as you request. PTAL👀 if you have time.

[hbelmiro]

/lgtm

cc @andreyvelich

[andreyvelich]

We can merge it.
Thanks @Electronic-Waste!
/lgtm
/approve

[google-oss-prow]

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