Improve type inference in SimpleCollector.Builder

[andersschuller]

I have tweaked some of the generics on SimpleCollector and SimpleCollector.Builder in order to improve the type inference and compile-time guarantees when using builders. This should fix #65, but it actually also fixes an issue for Java users. Currently, the following code compiles, but fails at runtime with a ClassCastException:

Gauge gauge = Counter.build().name("name").help("help").register();

This is because the return types of register and create are currently inferred at the call site, rather than being determined by the builder itself. I have therefore added a second type parameter to the builder, which is the type of collector it builds.

I have also removed the second type parameter from SimpleCollector, since it was unused. I think the intention of this parameter was to use it as I am using the new builder type parameter, but since the builder classes are static this was not possible.

This change breaks the API for anyone extending SimpleCollector, but since the documentation says "you should never need to subclass this class", I'm assuming this is not really an issue.

[brian-brazil]

Thanks for this it looks good to me, @willfleury would you mind taking a look at this as you're more an expert here than I?

Are there any tests we could add to prevent an accidental regression here in the future?

This change breaks the API for anyone extending SimpleCollector, but since the documentation says "you should never need to subclass this class", I'm assuming this is not really an issue.

Indeed.

[willfleury]

Looks good. Much better than we had it originally

[brian-brazil]

Great, thanks!