Currently we are overwriting the RUST_BACKTRACE if set
explicitly by user while running performance metrics tests
using dev_cli.sh
This change will allow user to set the RUST_BACKTRACE
while running performance metrics tests with dev_cli.sh
which invokes run_metrics.sh to run the performance binary.
We will set RUST_BACKTRACE to 1 if not set explicitly.
Signed-off-by: smit-gardhariya <gardhariya.smit@gmail.com>