CreateMsg is the legacy function to create messages. It's not very fast
WritePointMsg is like CreateMsg, except optimized for MetricPoint and buffer re-use. caller must assure a cap-len diff of at least: 33B (for FormatMetricPoint) 29B (for FormatMetricPointWithoutOrg) no other formats supported.
identifier of message format
sets m.Metrics to a *schema.MetricData any subsequent call may however put different MetricData into our m.Metrics array
parses format and id (cheap), but doesn't decode metrics (expensive) just yet.