Add a monitoring of S2OPC event Queue size

Description

User application may need to have somehow an way to monitor S2OPC performance by measuring on internal event queue. Currently, the application only logs a warning (or a fatal stop) is raised when a threshold is reached (SOPC_MAX_NB_ELEMENTS_ASYNC_QUEUE / SOPC_MAX_NB_ELEMENTS_ASYNC_QUEUE_WARNING_ONLY)

Detailed description

My proposal is to add an access to the event Queue size. This seems sufficient so that an application can detect overrun and adapt its dynamic behavior rather that crashing.

Code version identification

N.A.

Security impact

No impact.

Implementation

(What should be done to implement the new feature. If several possibilities indicate pro/cons/limitations for each solution)

See https://gitlab.com/systerel/S2OPC/-/commits/jch-add-service-queue-monitoring (jch-add-service-queue-monitoring)

Known limitations

(Limitations of the new feature: scope of the feature and regarding specification)