mongodb: Metrics origin clarification

Problem: When working - on designing dashboards, it's very hard to understand where the metrics originated from. What command or what query is executed? Or even what part of the code - generated the metric.  

2) The inability to search for metrics or information about them makes it difficult to understand collected data.

A list of information that can help:

• command executed on DB to collect metrics

• where from the response structure the metric was located 

• line in code - generated the metric, so it can be investigated later 

Proposed solution: 

improve exporter code to generate label / help text with some information about metrics origin.