feat: Add messaging.batch.message_count

[JPeer264]

Description

closes #338

generated via yarn run create:attribute

Adds the OTel messaging.batch.message_count attribute, used by the Cloudflare JS SDK to record the size of a Queue batch.

PR Checklist

• I have run yarn test and verified that the tests pass.
• I have run yarn generate to generate and format code and docs.

If an attribute was added:

• The attribute is in a namespace (e.g. nextjs.function_id, not function_id)
• I have used the correct value for pii (i.e. maybe or true. Use false only for values that should never be scrubbed such as IDs)

If an attribute was deprecated:

• I've followed the policies described in CONTRIBUTING.md

[github-actions]

Semver Impact of This PR

🟡 Minor (new features)

📋 Changelog Preview

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New Features ✨

• (attributes) Add device.low_power_mode attribute for iOS by itaybre in #314

• Add messaging.batch.message_count by JPeer264 in #341

• Add db.redis.key by sentrivana in #326
• Add process.command_args by sentrivana in #327

Bug Fixes 🐛

• Don't backfill sentry.browser.* attributes by loewenheim in #339
• Backfill several deprecated attributes by loewenheim in #332

Internal Changes 🔧

• (attributes) Mark deprecated gen_ai and ai attributes as backfill by vgrozdanic in #333
• (deps) Bump astro from 6.0.2 to 6.1.6 in /docs by dependabot in #330

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