chore(launcher): revert metadata to custom properties & artifact name -> event path

[Bobgy]

Description of your changes:
Reasons:

• in the future, when MLMD supports filtering, only top level fields in custom properties will be supported, so we'd better keep using them
• move artifact name to event path field, because

// Event path is conceptually artifact name for the execution.
// We cannot store the name as a property of artifact "a", because for example:
// 1. In first task "preprocess", there's an output artifact "processed_data".
// 2. In second task "train", there's an input artifact "dataset" passed from "preprocess"
// task's "processed_data" output.
//
// Now the same artifact is called "processed_data" in "preprocess" task, but "dataset" in
// "train" task, because artifact name is related to the context it's used.
// Therefore, we should store artifact name as a property of the artifact's events
// (connects artifact and execution) instead of the artifact's property.

Checklist:

• The title for your pull request (PR) should follow our title convention. Learn more about the pull request title convention used in this repository.

[zijianjoy]

/lgtm

Thank you Yuan for the change!

[zijianjoy]

/test kubeflow-pipelines-samples-v2

The previous test's sub-pipeline seems finished successfully https://4e18c21c9d33d20f-dot-datalab-vm-staging.googleusercontent.com/#/runs/details/cbbb47bf-f409-4943-a2a9-387f55e942b8, but parent-pipeline didn't get the run status.

[google-oss-robot]

New changes are detected. LGTM label has been removed.

[Bobgy]

Thanks @zijianjoy! I fixed it

[Bobgy]

/approve
Getting the big changes in, feel free to keep reviewing this.

[google-oss-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: Bobgy

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• samples/OWNERS [Bobgy]
• v2/OWNERS [Bobgy]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment