SampleGen: generate one source code file per sample definition

[beccasaurus]

~ Placeholder issue ~

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Expected output of the sample generator is: a list of code source files, one for each region tag. The name of the file should == the region tag, when possible (language dependent).

This will require a default calling form which generates samples which match DPE's canonicals, 1 source code file per sample definition.

In Node.js, Python, Ruby, PHP, Go ~ the file name should be exactly region_tag.ext
Probably the same in C#, but PascalCase.
Java must be the class name, iirc.

#PLACEHOLDER

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@googleapis/samplegen

[yihanzhen]

We could make region tag the class name for Java?

[beccasaurus]

For now let's do region tag across the board. In Java/C# this'll mean the class name should == Region Tag (although not actually a language requirement for C#)

region_tag	speech_transcribe_v1beta1
C#	SpeechTranscribeV1beta1.cs
Go	speech_transcribe_v1beta1.go
Java	SpeechTranscribeV1beta1.java
Node.js	speechTranscribeV1beta1.js
PHP	speech_transcribe_v1beta1.go
Python	speech_transcribe_v1beta1.py
Ruby	speech_transcribe_v1beta1.rb

[vchudnov-g]

Let's implement this by making the filename be the region tag if we generate exactly one sample with that region tag. If, in a given language, we generate n>1 samples for the same region tag, let's disambiguate by appending the calling form to at least (n-1) samples in that language.

One idea is to disambiguate all of them with the suffix. Another idea is that if the default calling form is produced only once, that sample does not get a disambiguating suffix but the other samples do. (I like that latter idea, once we have implemented default calling forms!)

[yihanzhen]

Fixed by #2713