Create goBackwardCompatibleMetrics option that defaults to true

[ktrz]

As I was fixing #224 I've noticed that for Go benchmarks that output multiple metrics the action prior to v1.18.0 was extracting a unit that was slightly wrong. For this reason to keep backward compatibility I've kept this metric (PR #225) being calculated like that (in addition to the new way with multiple metric)

We should create an option that would default to keep this backward compatibility but would enable opting out of this behavior. In the next version it would get deprecated with a warning and then removed with only one way of extracting metrics

[ningziwen]

Just want to clarify: when goBackwardCompatibleMetrics is true, does it mean it will return to the old wrong approach, outputting single unit?

[ktrz]

My intention was to emit both old approach and new approach in case when goBackwardCompatibleMetrics is true. This way we make sure that existing users will still get the same data for their existing past benchmarks as well as the new data split by metric

[ktrz]

In #225 I've implemented the backward compatibility but in this issue I want to make that optional (with backward compatibility by default)

[ningziwen]

I don't have strong preference but adding the option seems not a normal approach to me. Too much overhead to maintain it.

Current state is good enough to me. We can just mark it deprecated and remove the support when we bump major version at some point.