Usage metering — PR text

Problem / feature description

You are opening a pull request for branch feature/usage-metering-hooks that wires the billing service to new usage events emitted by the data plane. The integration depends on a client bump that merged elsewhere: pull request #1847 in the separate billing-sdk repository (same org). Your squad also has an internal design note at go/billing-metering that reviewers should read for field semantics.

Ticket BILL-901 tracks the epic. Save the GitHub PR body as pr-body.md (description field only, not the title).

Output specification

Create exactly one file pr-body.md with the full Markdown for the pull request description.