Add index to non unique hwmon sensor names

[discordianfish]

This fixes #406.

Arguably this could be fix more efficient but code could be made more idiomatic but I think it's good enough for now.

[grobie]

@rtreffer can you take a look?

[SuperQ]

I'd rather name the sensor after the filename, like temp1 and temp2 since these will be unique.

[rtreffer]

Sorry I'm sick/in bed and can't touch this for the next few days. Am 7. Januar 2017 07:06:26 MEZ schrieb Tobias Schmidt <notifications@github.com>:

…

@rtreffer can you take a look? -- You are receiving this because you were mentioned. Reply to this email directly or view it on GitHub: #408 (comment)

-- Diese Nachricht wurde von meinem Android-Mobiltelefon mit K-9 Mail gesendet.

[rtreffer]

Looks reasonable. Am 7. Januar 2017 07:06:26 MEZ schrieb Tobias Schmidt <notifications@github.com>:

…

@rtreffer can you take a look? -- You are receiving this because you were mentioned. Reply to this email directly or view it on GitHub: #408 (comment)

-- Diese Nachricht wurde von meinem Android-Mobiltelefon mit K-9 Mail gesendet.

[grobie]

The longer I think about it, the more I agree with @SuperQ on using the filename instead. Introducing our own index means users will need to understand the implementation in node_exporter in order to match our names to the their system.

I just checked and many of my sensors don't provide a <sensor>_label file. I'm not it's a good idea to change the value of the sensor label depending on whether a label is available or not, it should be always the same.

Maybe @brian-brazil has another idea, but one option would be to always use the sensor file prefix (temp1, fan2, etc.) as sensor label value and introduce a new metric like node_hwmon_sensor_label with the labels hwmon, sensor, label to export the human readable labels where available.

[grobie]

As we use the hwmon name as label as well, the index structure should be relative to the hwmon group. There is no need to attach a custom index to a sensor name in hwmon4 if there exists a sensor with the same label in hwmon3.

[brian-brazil]

Filename sounds the most practical here.

[discordianfish]

@grobie I wish you thought about those things when reviewing the initial PR.. I'll see if I can find an easy fix, otherwise I'm going to revert the whole hwmon collector since right now it breaks the node-exporter.