Refactor experiments local var naming

[pda]

Two parts, both rather superficial…

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Simplify the implementation of experiments.IsEnabled(). Go's map type returns the zero-value of its value type for missing keys; so map[string]bool returns false for missing keys.

That means we don't need any extra logic to check for the existence of the key.

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The rest of the PR is mostly var renaming to make the implementation of Enabled() clearer.

Idiomatic Go uses ok to indicate whether a key existed in a map, e.g. value, ok := experiments["foo"].
Whereas we were using it to refer to the boolean value of the map[string]bool in the context of a range,
i.e. for key, ok := range experiments.

And var enabled []string as the accumulator list of enabled experiments is problematic because enabled isn't exclusively plural; the same enabled name could also refer to a boolean value in the experiments map[string]bool. So this patch renames the list to keys, and also uses key instead of experiment everywhere to refer to experiment keys.

With the name enabled no longer being used for a list of enabled keys, this patch uses enabled instead of ok, resulting in a clearer if enabled { ... }.

[yob]

Thanks for the great PR description, I learnt some things.

The changes looks reasonable 👍