[Diagnostics] Datasources: number of unacknowledged messages, number of rate limited events

[avtolstoy]

submission notes

**Important:** Please sanitize/remove any confidential info like usernames, passwords, org names, product names/ids, access tokens, client ids/secrets, or anything else you don't wish to share.

Please Read and Sign the Contributor License Agreement ([Info here](https://github.com/spark/firmware/blob/develop/CONTRIBUTING.md)).

You may also delete this submission notes header if you'd like. Thank you for contributing!

Solution

Implements:

• Datasource for the number of unacknowledged CoAP messages (DIAG_ID_CLOUD_UNACKNOWLEDGED_MESSAGES)
• Datasource for the number of events dropped due to rate limiting (DIAG_ID_CLOUD_RATE_LIMITED_EVENTS)

Steps to Test

N/A

Example App

N/A

References

• [CH7640]
• [CH7641]

---

Completeness

• User is totes amazing for contributing!
• Contributor has signed CLA (Info here)
• Problem and Solution clearly stated
• Run unit/integration/application tests on device
• Added documentation
• Added to CHANGELOG.md after merging (add links to docs and issues)

[m-mcgowan]

Good changes 👍 , please fix up the conflict so we can merge

[m-mcgowan]

this has been merged into develop