Ensures pending counter in rabbit_shovel_status is always an integer (backport #14614)

[mergify]

Proposed Changes

We observed that when a shovel connection for AMQP091 run into blocked state, pending messages stack up in a list of messages, which leads to the response of rabbitmq_shovel_status:status() can get very large and can cause memory leaks with many shovels (or one shovel using no-ack). It also breaks the management UI for shovel status.

steps to reproduce:

• create a shovel from cluster-a to cluster-b.
• simulate memory watermark on cluster-b.
• shovel a couple messages, navigate to shovel status. should now break with 500 internal error. rabbitmq_shovel_status:status() on the source will show pending as a list of messages.
• remove memory watermark, see messages flow again.
• shovel status will still be 500, and rabbitmq_shovel_status:status() will show #{pending => {[],[]},

This PR ensures that the number of pending messages is an integer for all shovel protocols. I have tried confirming if the bug exist in AMQP1.0, however I get other issues then which I am currently investigating.

Types of Changes

What types of changes does your code introduce to this project?
Put an x in the boxes that apply

• Bug fix (non-breaking change which fixes issue )
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause an observable behavior change in existing systems)
• Documentation improvements (corrections, new content, etc)
• Cosmetic change (whitespace, formatting, etc)
• Build system and/or CI

Checklist

Put an x in the boxes that apply.
You can also fill these out after creating the PR.
This is simply a reminder of what we are going to look for before merging your code.

• Mandatory: I (or my employer/client) have have signed the CA (see https://github.com/rabbitmq/cla)
• I have read the CONTRIBUTING.md document
• I have added tests that prove my fix is effective or that my feature works
• All tests pass locally with my changes
• If relevant, I have added necessary documentation to https://github.com/rabbitmq/rabbitmq-website
• If relevant, I have added this change to the first version(s) in release-notes that I expect to introduce it

Further Comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution
you did and what alternatives you considered, etc.

---

This is an automatic backport of pull request #14614 done by [Mergify](https://mergify.com).