fix(microservices): cleanup unary call on unsubscribe

[szilveszterandras]

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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?

For grpc unary requests, if the consumer unsubscribes from the Observable before the unary response arrives, the call isn't cancelled, as opposed to a streaming endpoint, which does handle client-side unsubscribes. This is a problem in high-performance grpc applications, where we may build timeout/retry logic into demanding endpoints, such as long-running db queries. Retried but not closed upstream calls can lead to unnecessary cpu load, network overhead, etc.

I provided a minimal reproduction example here: https://github.com/szilveszterandras/nest-unary

Issue Number: N/A

What is the new behavior?

If a subscriber unsubscribes from the Observable coming out of a unary request, the grpc call is cancelled.

Does this PR introduce a breaking change?

• Yes
• No

Other information

[coveralls]

Pull Request Test Coverage Report for Build 7693fc3d-16d8-485b-83dd-adb7b7a513fb

• 4 of 4 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.003%) to 93.82%

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Totals
Change from base Build 33d64ea6-8564-4074-89ae-b787d2d45c2d:	0.003%
Covered Lines:	6103
Relevant Lines:	6505

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💛 - Coveralls

[kamilmysliwiec]

lgtm