-Add -g option to save sample indices following Go's PGO requirements

[insilications]

When converting perf.data, PerfDataConverter::GetOrCreateBuilder creates the event name using event_type.name(), adding an underscore and saving it in event_name. It then sets the type of the sample to event_name + "sample" (e.g. "cycles:u_sample").

The problem is that when using the converted perf.data with Go's PGO, it complains that the profile should have at least one of the indices with type/unit of samples/count or cpu/nanoseconds (https://go.dev/doc/pgo#alternative-sources). This adds the -g option to save the sample indices following Go's PGO requirements.

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[insilications]

Signed the CLA. I have no idea if perf_data_converter is the place to add support for this use case, but it could help people who are relying on perf_data_converter to create PMU profiles for use with Go's PGO.