Report SQL server correctly

Fixes for the problem that @tmichelet was trying to fix with #968.

What the issue is: Most of the metrics we report from SQL Server are not reported correctly

How to fix it: [Microsoft documentation](http://blogs.msdn.com/b/psssql/archive/2013/09/23/interpreting-the-counter-values-from-sys-dm-os-performance-counters.aspx) explains how to do it properly. Each counter has associated with him a value `cntr_type` explaining how to report it. We can read this value and report the metric accordingly. This is preferred to setting the report method in advance because some metrics may change type between different verisons / OS 

Implementation info: In order to not break existing setup of users, the "type" (rate, gauge, histogram) attributes in the config is supported and overwrite any determination that we do ourselves.

@tmichelet @conorbranagan if you want to take a look.
You can unpack that directly in the for statement:
```python
for name, counter_name, instance_name in self.METRICS:
```