add helper.go and tests

[huyan0]

here is a link to the overall project design

helper.go contains helper functions used to convert OTLP metrics into TimeSeries. It has the following methods:

• validateMetrics: returns a bool representing whether the metric has a valid type and temporality combination.

• addSample: add a sample to its corresponding TimeSeries to the map based on a label set

• createLabelSet: creates a slice of Cortex Label with OTLP labels and paris of string values.

• getPromMetricName: creates a Prometheus metric name from OTLP metric

• timeSeriesSignature: get a unique string signature for a OTLP metric

helper.go is used by cortex.go. PR #7 contains both files.