Add getter for TelemetryTracer

[Thidasala]

In order to facilitate accessing the TelemetryTracer from a custom class mediator, we have to provide a utility method that resides in the org.wso2.carbon.apimgt.gateway package and returns the TelemetryTracer, since we don't expose org.wso2.carbon.apimgt.gateway.internal.ServiceReferenceHolder from the org.wso2.carbon.apimgt.gateway bundle.

[CLAassistant]

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[wggihan]

@Thidasala @senthuran16 Please add a proper description for this issue.