[Accessibility]: Announce file decoration <meter> element aria-label

Created by: taras-yemets

Closes https://github.com/sourcegraph/sourcegraph/issues/36857

Part of https://github.com/sourcegraph/sourcegraph/issues/36751

Test plan

• sg start web-standalone
• follow steps to replicate the user journey (ensure codecov.fileDecorations.show setting is enabled)
• follow the instructions to audit a user journey
• ensure meter element hover message is announced by the screen reader

App preview:

• Web
• Storybook

Check out the client app preview documentation to learn more.