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

content/en/docs/components/katib/user-guides/metrics-collector.md

@@ -6,16 +6,23 @@ weight = 40
 
 This guide describes how Katib metrics collector works.
 
-## Metrics Collector
+## Overview
+
+There are two ways to collect metrics:
+
+1. Pull-based: collects the metrics using a _sidecar_ container. A sidecar is a utility container that supports
+the main container in the Kubernetes Pod.
+
+2. Push-based: users push the metrics directly to Katib DB in the training scripts.
 
 In the `metricsCollectorSpec` section of the Experiment YAML configuration file, you can
 define how Katib should collect the metrics from each Trial, such as the accuracy and loss metrics.
 
-Your training code can record the metrics into `StdOut` or into arbitrary output files. Katib
-collects the metrics using a _sidecar_ container. A sidecar is a utility container that supports
-the main container in the Kubernetes Pod.
+## Pull-based Metrics Collector
 
-To define the metrics collector for your Experiment:
+Your training code can record the metrics into `StdOut` or into arbitrary output files. 
+
+To define the pull-based metrics collector for your Experiment:
 
 1. Specify the collector type in the `.collector.kind` field.
    Katib's metrics collector supports the following collector types:
@@ -29,7 +36,7 @@ To define the metrics collector for your Experiment:
      metrics must be line-separated by `epoch` or `step` as follows, and the key for timestamp must
      be `timestamp`:
 
-     ```
+     ```json
      {"epoch": 0, "foo": "bar", "fizz": "buzz", "timestamp": "2021-12-02T14:27:51"}
      {"epoch": 1, "foo": "bar", "fizz": "buzz", "timestamp": "2021-12-02T14:27:52"}
      {"epoch": 2, "foo": "bar", "fizz": "buzz", "timestamp": "2021-12-02T14:27:53"}
@@ -51,9 +58,6 @@ To define the metrics collector for your Experiment:
      in the `.collector.customCollector` field. Check the
      [custom metrics collector example](https://github.com/kubeflow/katib/blob/ea46a7f2b73b2d316b6b7619f99eb440ede1909b/examples/v1beta1/metrics-collector/custom-metrics-collector.yaml#L14-L36).
 
-   - `None`: Specify this value if you don't need to use Katib's metrics collector. For example,
-     your training code may handle the persistent storage of its own metrics.
-
 2. Write code in your training container to print or save to the file metrics in the format
    specified in the `.source.filter.metricsFormat` field. The default metrics format value is:
 
@@ -79,3 +83,46 @@ To define the metrics collector for your Experiment:
    recall=0.55
    precision=.5
    ```
+
+## Push-based Metrics Collector
+
+Your training code needs to call [`report_metrics()`](https://github.com/kubeflow/katib/blob/e251a07cb9491e2d892db306d925dddf51cb0930/sdk/python/v1beta1/kubeflow/katib/api/report_metrics.py#L26) function in Python SDK to record metrics. 
+The `report_metrics()` function works by parsing the metrics in `metrics` field into a gRPC request, automatically adding the current timestamp for users, and sending the request to Katib DB Manager. 
+
+But before that, `kubeflow-katib` package should be installed in your training container. 
+
+To define the push-based metrics collector for your Experiment, you have two options:
+
+- YAML File
+
+    1. Specify the collector type `Push` in the `.collector.kind` field.
+
+    2. Write code in your training container to call `report_metrics()` to report metrics.
+
+- [`tune`](https://github.com/kubeflow/katib/blob/master/sdk/python/v1beta1/kubeflow/katib/api/katib_client.py#L166) function
+
+    Use tune function and specify the `metrics_collector_config` field. You can reference to the following 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(namespace="kubeflow").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"].
+      # Currently, the training container should have `git` package to install this SDK. 
+      packages_to_install=["git+https://github.com/kubeflow/katib.git@master#subdirectory=sdk/python/v1beta1"],
+    )
+    ```