fix(common,core): auto flush logs after useLogger call

[micalevisk]

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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)
• 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: closes #8733

I tested it locally with the following usages -- assuming that there was a call to app.useLogger:

NestFactory.createApplicationContext(AppModule)

flushes (even tho the aren't logs buffered).

NestFactory.createApplicationContext(AppModule, { bufferLogs: true })

flushes.

NestFactory.createApplicationContext(AppModule, { bufferLogs: true, autoFlushLogs: true })

flushes.

NestFactory.createApplicationContext(AppModule, { bufferLogs: true, autoFlushLogs: false })

do not flushes.

What is the new behavior?

When using NestApplicationContext app and autoFlushLogs is on, invoking useLogger method on it will flushes its logs.

Does this PR introduce a breaking change?

• Yes
• No

Other information

A naive attempt to address that issue without much effort. I have few concerns regarding my solution but I didn't come with anything better (nor easy to follow). Feel free to rejected it then :p

[coveralls]

Pull Request Test Coverage Report for Build a668766f-3841-414a-ad8a-9f83e671dd53

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 94.08%

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Totals
Change from base Build b0bb0a6e-71e0-4feb-a373-e0a70f8ad93b:	0.0%
Covered Lines:	5658
Relevant Lines:	6014

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💛 - Coveralls

[jmcdo29]

Happy to see this added in. Left a minor comment, but the functionality itself looks fine to me

[kamilmysliwiec]

LGTM