Add realtime billing reporters

[russellmartin-livekit]

Summary

• expose generated reporters for billable realtime text-input messages
• expose generated reporters for discounted cached-image tokens
• keep both dimensions distinct from operational request counts and gross image input

Test plan

• run go test ./observability/gatewayobs
• verify generated reporter and no-op implementations

[changeset-bot]

⚠️ No Changeset found

Latest commit: 18f0a1b

Merging this PR will not cause a version bump for any packages. If these changes should not result in a new version, you're good to go. If these changes should result in a version bump, you need to add a changeset.

This PR includes no changesets

When changesets are added to this PR, you'll see the packages that this PR includes changesets for and the associated semver types

Click here to learn what changesets are, and how to add one.

Click here if you're a maintainer who wants to add a changeset to this PR