[fix][fn] bounds check for metrics family

[flowchartsman]

Fixes apache#20020

Modifications

check for len() == 0 before iterating, otherwise return empty map.

Verifying this change

• Make sure that the change passes the CI checks.

This change added tests and can be verified as follows:

• *New test exercises failure condition (panics without fix), and tests instance communication besides. Also adds a helper for similar tests, should they need it.

Does this pull request potentially affect one of the following parts:

If the box was checked, please highlight the changes

• Dependencies (add or upgrade a dependency)
• The public API
• The schema
• The default values of configurations
• The threading model
• The binary protocol
• The REST endpoints
• The admin CLI options
• The metrics
• Anything that affects deployment

Documentation

• doc
• doc-required
• doc-not-needed
• doc-complete