Rudy/golang wildcard

Add wildcard support for the golang_expvar.
The wildcard can be used for a replacement of any key/index, so that it reports everything.
I think there is an issue with the regex here, if I am right it would be `'^*$'` which is not a valid regex. Second question is do we need to use `re` for that?
Something like
```python
if key == '*' or key == new_key
```