Add support to generate metrics queries

We have know Usage Ping instrumentation classes !57333 (closed)

We should add a method to get the generated queries

Available methods are

metric = Gitlab::Usage::Metrics::Instrumentations::Issues::ByStageMonthlyPlanMetric.new

metric.value 
=> 0

Proposal

Have a method which returns the information corresponding to implementation

• For non database metrics will return the actual value value
• For database metrics will return generated sql