Fix OpenTelemetry graceful shutdown.

When running OPA with the distributed tracing option enabled,
the OpenTelemetry trace exporter is not gracefully shut down
when the server is stopped.

This PR fixes that issues by moving the trace exporter shutdown
in the gracefulServerShutdown function.

Fixes: #6651
Signed-off-by: Nicolas Chotard <nicolas.chotard@backmarket.com>
Signed-off-by: David Wobrock <david.wobrock@backmarket.com>