[BEAM-2163] Remove the dependency on examples from ptransform_test

[jglezt]

The test_read_metrics function no longer depends on the snippets file. The CountingSource class is contained in io.utils which forms part of the library.

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[pabloem]

Hello Javier.
Since CountingSource might be used in other cases (e.g. other tests), could you add it to beam/sdks/python/apache_beam/io/ instead?

[jglezt]

It makes sense to put the class on the iobase.py or is preferable to create a single file for this class?

[pabloem]

I was trying to find a file that it's appropriate for. I'm thinking that a new file should be fine for it.

[jglezt]

Hi Pablo
I put the class on a general snippets file inside the io folder. I put named the file snippets in case any other function or class that is required on the io folder, but does not makes sense under any other file.

[pabloem]

Hi Javier!
I'm sorry about the back-and-forth. Could you rename the file to 'utils' instead of 'snippets', and remove the [START model_custom_source_new_source] and [END model_custom_source_new_source]?
Thanks!

[jglezt]

Hi Pablo
Thanks for the feedback : )
Changed file name and removed comments as suggested.