Add confluent kafka producer poll and flush returns

[dferrochio]

Description

Add return statement to Confluent kafka Producer poll() and flush() calls when instrumented by ConfluentKafkaInstrumentor().instrument_producer()

Fixes #2525

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

Added unittests for instrumented Confluent kafka producer poll and flush methods.

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: lzchen / name: Leighton Chen (d939628)
• ✅ login: dferrochio (a333099, aaccb96, 17144fa, 3df70cd, 88b372d, 23952d4)
• ✅ login: ocelotl / name: Diego Hurtado (967233e)
• ✅ login: xrmx / name: Riccardo Magliocchetti (7df76e3)

[xrmx]

@dferrochio You need to sign the CLA

[dferrochio]

@xrmx CLA signed

[dferrochio]

@oxeye-dorkolog can review?, please