Store clean metric name separately

[npazosmendez]

This idea came out of the following discussion: #686 (comment)

The problem

Some functions need the name of the metric clean from function names (usually to split it into nodes). For example:

carbonapi/expr/helper/helper.go

Lines 95 to 96 in b1e09e9

	metricTags := arg.Tags
	nodes := strings.Split(metricTags["name"], ".")

The problem is that the "name" is not always clean from functions. For example: https://github.com/graphite-project/graphite-web/blob/ed08fa4e2f6526ae11e55f88f70d0b6d36e4052f/webapp/graphite/render/functions.py#L243-L249 . In that case, strings.Split("sumSeries(foo.bar)", ".") returns "sumSeries(foo", "bar)", which is not the wanted nodes.

The first (currently implemented) solution for this is to call ExtractNameTag when the clean metric name is needed: https://github.com/grafana/carbonapi/blob/7b2a1ee8bfcba3bb1aba9619dc2f39d3cf9622c0/expr/helper/helper.go#L95-L96 (at time of writing, this is close to being merged here). But we want to avoid calling this function because:

• It can affect performance and it's avoidable
• The function is complex and error-prone, especially when all symbols are supported in metric names

The solution

The proposed solution is to have a separate private field in the MetricData struct to store the metric name clean from function names, so it's taken from there whenever it is needed.