feat: automatically convert Vec into HashMap in histogram_opts! macro

[chad-codes]

I noticed that this happens in the opts! macro already so I think it makes sense for histogram_opts! to also do this.

[lucab]

Thanks for the PR! CI does not seem to like the proposed change, but I didn't look deeper into what's going on. On which toolchain version are you developing this? Does cargo test succeed for you locally?

[chad-codes]

Whoops, I had made the original commit directly from Github without cloning and testing. I've updated the tests and everything is working.

This is a breaking change to existing functionality since histogram_opts! now expects labels to be passed in like labels! {"key" => "value"} instead of labels! {"key".to_string() => "value".to_string()}. This is how opts! works so I think it's an acceptable breaking change.