QAN : Save data after connection is broken with server

User story:
As a PMM user, I need to get all QAN/ related data filled in PMM when the connection between PMM Client and Server disappeared for some period of time so that I can investigate outages better.

UI/UX:
N/A

Acceptance criteria

• In case of a lost connection all QAN data for up to 15 minutes should be available

Out of scope:

Suggested implementation:

Implement cyclic buffer on PMM agent side with configurable size. (Should be discussed)

How to test:

1. QAN test

   1. setup PMM Server and pmm-agent on different instances.

   2. Somehow simulate network disconnect.

   3. Wait for 10 minutes

   4. Reconnect

   5. Check QAN metrics

Details:
TBD