Only enable OpenTelemetry when explicitly requested

[tom-binary]

Avoid loading or calling OpenTelemetry unless explicitly enabled, e.g. via

USE_OPENTELEMETRY=1 myriad.pl ...

This is due to stability problems relating to dynamically and cancellation/exception handling: especially on recent versions of Perl, we're seeing various memory corruption issues. Since this is supposed to be an opt-in feature, due to performance hit and that it's not useful without the OpenTelemetry endpoint and infrastructure available, disabling by default was the intended direction anyway.

Symptoms include:

Attempt to free unreferenced scalar: SV 0x5603c40210f8 at /opt/perl-5.40.0/lib/site_perl/5.40.0/OpenTelemetry/Context.pm line 9.

and segmentation faults.

This change attempts to bypass OpenTelemetry (including module load) unless the USE_OPENTELEMETRY environment variable is set. Previously, we'd be creating contexts and spans in some places regardless of whether it was enabled.