[fix][fn] check user metric len before iterating

[flowchartsman]

Fixes #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

Matching PR in forked repository

PR in forked repository: flowchartsman#1

[github-actions]

@flowchartsman Please add the following content to your PR description and select a checkbox:

- [ ] `doc` <!-- Your PR contains doc changes -->
- [ ] `doc-required` <!-- Your PR changes impact docs and you will update later -->
- [ ] `doc-not-needed` <!-- Your PR changes do not impact docs -->
- [ ] `doc-complete` <!-- Docs have been already added -->

[tisonkun]

LGTM

Thanks for taking care of this!

[codecov-commenter]

Codecov Report

Merging #20021 (be67293) into master (3b118b6) will increase coverage by 0.00%.
The diff coverage is 44.60%.

@@            Coverage Diff            @@
##             master   #20021   +/-   ##
=========================================
  Coverage     72.86%   72.86%           
- Complexity    31628    31634    +6     
=========================================
  Files          1861     1858    -3     
  Lines        137500   137424   -76     
  Branches      15141    15107   -34     
=========================================
- Hits         100187   100136   -51     
+ Misses        29351    29301   -50     
- Partials       7962     7987   +25     

Flag	Coverage Δ
inttests	24.35% <0.95%> (-0.05%)	⬇️
systests	24.87% <20.14%> (-0.15%)	⬇️
unittests	72.15% <35.49%> (+0.02%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
...a/org/apache/pulsar/broker/admin/v2/Functions.java	0.00% <0.00%> (ø)
...java/org/apache/pulsar/broker/admin/v2/Worker.java	11.76% <0.00%> (ø)
...org/apache/pulsar/broker/admin/v2/WorkerStats.java	40.00% <0.00%> (ø)
...ons/worker/rest/api/v2/FunctionsApiV2Resource.java	0.00% <0.00%> (ø)
...s/worker/rest/api/v2/WorkerStatsApiV2Resource.java	0.00% <0.00%> (ø)
...ulsar/functions/worker/rest/api/FunctionsImpl.java	56.55% <16.66%> (-0.74%)	⬇️
...org/apache/pulsar/broker/admin/impl/SinksBase.java	35.29% <35.71%> (ø)
...g/apache/pulsar/broker/admin/impl/SourcesBase.java	35.29% <35.71%> (-1.85%)	⬇️
...ons/worker/rest/api/v3/FunctionsApiV3Resource.java	43.39% <37.50%> (ø)
...e/pulsar/functions/worker/rest/api/WorkerImpl.java	14.90% <38.46%> (+9.38%)	⬆️
... and 15 more

... and 65 files with indirect coverage changes

[tisonkun]

Merging...

@flowchartsman Thanks for your contribution! You can ping me as a reviewer the next time for usability changes.