refactor(microservices): improve speed of grpc server initialization

[yjiq150]

PR Checklist

Please check if your PR fulfills the following requirements:

• 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)
• Docs have been added / updated (for bug fixes / features)

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: N/A

In order to find grpc service descriptors, collectDeepServices function recursively searches whole grpcDefinitions generated by protoLoader, which currently includes all descriptors like namespace, service, and field.

What is the new behavior?

Because field descriptors cannot have namespace as their child, they don't need to be recursively searched. They can distinguished by checking existence of 'format' field and they can be skipped from the search to speed up the process.

Does this PR introduce a breaking change?

• Yes
• No

Other information

In the project I am working on, proto models are heavily used.
It took more than 3 seconds in M2 MacBook Pro to run collectDeepServices before the modification.
Now it takes around 300ms in the same environment after the modification

[coveralls]

Pull Request Test Coverage Report for Build 9a0d37e7-42c8-4e54-a3ae-9b3a5c95c083

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.002%) to 92.125%

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Totals
Change from base Build eacd3e56-4bed-4f5e-9bab-412a10335aab:	0.002%
Covered Lines:	6738
Relevant Lines:	7314

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💛 - Coveralls

[kamilmysliwiec]

lgtm