Lack of consistency in retrieval of observed values

[iustin]

Hi,

Looking at the library, it looks like:

• Counter and Gauge allow retrieving the current value.
• Family half-allows it - there's get_or_create, but that has the side-effect of creating the value if non-existent.
• Histogram and Info don't allow it at all.

Now, in production, there's not much use, but in tests, I'd rather be able to test the values directly rather than encode and parse back the text format. Would you mind a pull request allowing clean retrieval of:

• Info string
• Histogram count and sum?

Thanks!