feat(microservices): return all clients from kafka unwrap

[kostyazgara]

PR Checklist

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• The commit message follows our guidelines: https://github.com/nestjs/nest/blob/master/CONTRIBUTING.md
• Tests for the changes have been added (for bug fixes / features)
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PR Type

What kind of change does this PR introduce?

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• Build related changes
• CI related changes
• Other... Please describe:

What is the current behavior?

Issue Number: #11616

While the new major release ships an ability to unwrap underlying server instance, it's still not possible to manipulate on existing consumer and producer instances which are created in start method. As an example provided in the issue, we still can't subscribe heartbeat event on consumer created by NestJS

What is the new behavior?

Following the example in Redis server, we can return all created instances, but not just client instance. In this case everyone can get an access to consumer and producers

Does this PR introduce a breaking change?

• Yes
• No

unwrap method is not type-safe and fully depend on developer who define expected type. If someone is already start using unwrap method we might get into the trouble of unexpected result and developer must change their expected type manually

const client = microservice.unwrap<Kafka>(); // old
const [client] = microservice.unwrap<[Kafka]>() // new

Other information

[coveralls]

Pull Request Test Coverage Report for Build a7d5ea97-fe6b-459e-b968-9201888555e4

Details

• 0 of 1 (0.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 89.696%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/microservices/server/server-kafka.ts	0	1	0.0%

Totals
Change from base Build 328c39aa-d23d-4fdf-994c-d2c72f2f696a:	0.0%
Covered Lines:	7103
Relevant Lines:	7919

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💛 - Coveralls