added custom metric sample

[GSmithApps]

What was changed

Added a custom metric sample

Why?

Help folks with questions about custom metrics

Checklist

1. How was this tested:
   Checked the metric output against the existing metric

2. Any docs updates needed?
   Updated main python samples readme

[GSmithApps]

This is failing on the black formatter, but I formatted and sorted the imports

[Copilot]

Pull Request Overview

This PR adds a custom metric sample to demonstrate how to record the workflow type in the activity schedule-to-start latency.

• Added a new workflow (ExecuteActivityWorkflow) and associated client code for executing activities.
• Introduced a custom activity interceptor to record custom metrics in the activity worker.
• Updated README files to include instructions for running the custom metric components.

Reviewed Changes

Copilot reviewed 6 out of 6 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
custom_metric/workflow_worker.py	Introduces a simple workflow that executes an activity.
custom_metric/client.py	Provides a client to start the new workflow.
custom_metric/activity_worker.py	Implements an interceptor for recording custom metrics for activity timing.
custom_metric/README.md	Updates sample usage instructions for the custom metric sample.
README.md	Adds the custom_metric sample to the main project documentation.

[cretz]

This sample is not just demonstrating custom metric, which can be done inside the activity, it's demonstrating interceptors. May be a little confusing to someone just wanting to understand the custom metric part.

[GSmithApps]

Thanks for the suggestion! I implemented it because what you mentioned off-PR, I thought you suggested to do this with an interceptor

[cretz]

For this specific custom metric that makes sense, but people can create custom metrics without interceptors. I think it's fine to stay, may just need to clarify in README via that one/two-sentence explainer that the purpose of this sample is to demonstrate a custom schedule to start metric via interceptor, as opposed to just general custom metric.

[GSmithApps]

I added a little blurb. I hope it's along the lines you're thinking

[cretz]

Can you add a test or two? (I know not all samples have tests, something we regret, but adding them helps us catch if they start breaking)

[GSmithApps]

Added!

[GSmithApps]

@cretz thanks for the review! I think all your comments have been acknowledged and fixed

[cretz]

Just minor stuff. Only thing really needed I think is to give the workflow its own file. Don't forget to poe format.

[cretz]

Looks great, thanks!