fix: add retry to google.auth.compute_engine._metadata.get()

[DaniilAnichin]

Initial fix of issue #211 was done in CL #323, but only for .ping()
This one is adding same behavior & tests for .get() method, as the problem still arises

Resolves: #211

[DaniilAnichin]

@dhermes can you take a look to finally make the #211 fixed?

[dhermes]

cc @busunkim96

@DaniilAnichin I am no longer maintaining this library. Cheers

[lucasmbrown]

I can't tell who the maintainers currently are, but @theacodes or @busunkim96, would you be available to review this? It's caused us a few headaches so far (right now we're wrapping all our gets in our own retry logic). Thanks!

[busunkim96]

@lucasmbrown Thank you for the ping! I missed this PR. I'll take a look now.

[busunkim96]

It looks like there's a mismatch between the actual retry count (5) and the count in the test (3). Otherwise this looks good to me. Thank you for the PR!

=================================== FAILURES ===================================
______________________ test_get_failure_connection_failed ______________________

    def test_get_failure_connection_failed():
        request = make_request("")
        request.side_effect = exceptions.TransportError()

        with pytest.raises(exceptions.TransportError) as excinfo:
            _metadata.get(request, PATH)

        assert excinfo.match(r"Compute Engine Metadata server unavailable")

        request.assert_called_with(
            method="GET",
            url=_metadata._METADATA_ROOT + PATH,
            headers=_metadata._METADATA_HEADERS,
        )
>       assert request.call_count == 3
E       AssertionError: assert 5 == 3
E        +  where 5 = <MagicMock spec='Request' id='140641503619536'>.call_count

tests/compute_engine/test__metadata.py:207: AssertionError

[DaniilAnichin]

Updated call count in tests; ping me if I need to rebase to current master