Update logs usage on troubleshooting guide

Since `GRPC_VERBOSITY=debug` is not enough to see the logs, it is needed to update the troubleshooting guide to include `GRPC_TRACE=all`.

Related issues: https://stackoverflow.com/a/60935367

Something like:

```
## GRPC_VERBOSITY

`GRPC_VERBOSITY` is used to set the minimum level of log messages printed by gRPC (supported values are `DEBUG`, `INFO` and `ERROR`). If this environment variable is unset, only `ERROR` logs will be printed.

**Note**: This environment variable only works if `GRPC_TRACE` is set.

## GRPC_TRACE

`GRPC_TRACE` can be used to enable extra logging for some internal gRPC components. Enabling the right traces can be invaluable
for diagnosing for what is going wrong when things aren't working as intended. Possible values for `GRPC_TRACE` are listed in [Environment Variables Overview](doc/environment_variables.md).
Multiple traces can be enabled at once (use comma as separator).

# Enable debug logs for an application
GRPC_VERBOSITY=debug GRPC_TRACE=all ./helloworld_application_using_grpc

```