Enable users to distinguish when there is queue length data from when the queue length size is 0

[tmasternak]

Description

When no queue length measurements are done in a given time bucket for a queue, the value shown for a given bucket (point on the graph) is zero.

This might be confusing for customers as in such scenarios the graph becomes "choppy", as captured in the description of:

• SQL Server and PostgreSQL can indicate report 0 as the queue length value ServiceControl#4556

The solution to the above issue mitigated this problem for SQL transports but the problem is still for example when querying a single queue takes longer, if there are many queues that are monitored etc.

Possible solutions

• SQL Server and PostgreSQL can indicate report 0 as the queue length value ServiceControl#4557 (comment)
• make sure that the monitoring instance keeps snapshot values for queue length for all monitored queues i.e. return the last measured value