Document how to read stored subscriptions for governance, metrics, etc

## What is the Problem Being Solved?

https://github.com/Agoric/agoric-sdk/pull/5678#issuecomment-1168804831

## Description of the Design

It should be a [documentation snippet](https://github.com/Agoric/documentation/blob/main/CONTRIBUTING.md#importing-and-testing-code-snippets).

Maybe also integrate with the ETL.

## Security Considerations

## Test Plan

