fix: flush reporter instead of closing

[cawolf]

During the dev_v3 development, there was a change in handling flush() for reporters: instead of using flush(), now close() was used.

This leads to the fact that you can use flush() on the tracer only once, which is a change which breaks e.g. our rabbitmq bundle.

We reintroduced flush() for reporters and use it instead of close().

[MollocH]

LGTM