Allow Simple/Batch processor to concurrently invoke Exporter.Export()

[cijothomas]

What are you trying to achieve?

Current spec restricts Simple/Batch processors from calling exporter.export() concurrently. This can be limiting in some scenarios:

1. When Exporting to Windows ETW, Linux user_events etc., the exporters can handle concurrent calls. This is important to get highest performance. These exporters are to be paired with SimpleProcessor, as they cannot afford to have the contention that is typical in the BatchProcessors. However, without SimpleProcessor concurretly calling export(), we wont get the required perf due to contention introduced to syncronize export() calls.
2.  Various discussions in Clarify spans export concurrency #4205 shows the need for concurrently calling export() from BatchProcessors as well.

In short, there is need for allowing both Simple,Batch processors to call export() concurrently.

To keep back-compatibility, we can do something like:

1. Allow exporters to advertise if their export() methods are okay to be called concurrently. If no explicit advertisement, then default to false.
2. Relax restriction in exporting processors (Simple/Batch), to call export() concurrently, if the paired exporter has advertised that they support being called concurrently.
3. Spec can be silent on "how" exporters advertise this, and leave it for implementations.

Additional context.

Previous attempt to make this happen, but only for SimpleProcessor: #4163

Opened for Logs, but equally applicable for Tracing too.