Fixed #22: do not send the first derivatives value

[IzabellaRaulin]

Fixes #22

Summary of changes:

• do not send the first derived values
• handle dynamic metrics in "CollectMetrics" in the model way
• added "calcGauge()" to set an value of pending_ops metric in separate place (before it was done in "calcDerivatives()" and do not skip this gauge-type metric in the first collection
• updated README.md:
  • added info about type of metric
  • added missing pending_ops metric to the metric list

How to verify it:

• run exemplary task manifest: in the first collection only gauge metric (_pendingops) is returned, the derived metrics are available from the second collection as expected

Testing done:

• legacy test
• run exemplary task manifest to verify that the first values of derive metrics are consistent

[candysmurf]

@IzabellaRaulin, the value of metric "/intel/procfs/disk/dm-0/octets_write" is pretty high comparing to other values. Do you mind double check to see if it's right? This is my rough look after I upgrade my system to Mac Sierra. We can sync up more tomorrow.

[candysmurf]

Let me take another look

[candysmurf]

@IzabellaRaulin, I'm probably very confused:-). I thought we only send derivatives including the first set of values. No raw data at this time. Raw data is for the future enhancement.

[IzabellaRaulin]

@candysmurf, I do not know whether I correctly understood what is no clear. Maybe I will explain that how it works:

• for every collection disk statistics are gathered (which are raw values) and stored in `dc.data; these data are needed to calculate derivatives
• for each collection except the first one derivatives are calculated (based on raw data stored in dc.data and dc.dataPrev) and the results from this calculation are kept in dc.output

There is only a one metric - pending_ops which is a gauge type, so this metric is returned as it is. The rest of currently available metrics are a derive type - see README.md

This plugin worked before in that way that is explained above, but after introduction of this line master/disk/disk.go#L212 the first derivatives values are sent as a raw, what broke consistency (https://github.com/intelsdi-x/snap-plugin-collector-disk/issues/22). This pull request fixes that :)

According:

No raw data at this time. Raw data is for the future enhancement.

Yes, that's right.

[candysmurf]

Thanks @IzabellaRaulin for details. LGTM