don't enforce dot after prefix

[Dieterbe]

primary use case: if prefix == "" my metrics shouldn't start with a dot

[mihasya]

I'd be worried about merging this, as anyone that's already running this with a prefix would suddenly have all their metrics change from foo.bar to foobar. Can you refactor this to be a bit more specific?

• If the prefix is set to "", don't add the dot.
• If the prefix is set to "foo", add a dot.
• If the prefix is set to "foo.", do nothing.

Thanks.