Add config.enable_tracing for easier performance tracing setup

[sl0thentr0py]

The new optional boolean will set traces_sample_rate to 1.0
automatically if not set yet. If false, this will also disable tracing
even if a sample rate or sampler is set.

closes #1996

[codecov]

Codecov Report

Patch coverage: 100.00% and no project coverage change

Comparison is base (a471812) 98.56% compared to head (2bf8dca) 98.56%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2005   +/-   ##
=======================================
  Coverage   98.56%   98.56%           
=======================================
  Files         157      157           
  Lines       10075    10105   +30     
=======================================
+ Hits         9930     9960   +30     
  Misses        145      145           

Impacted Files	Coverage Δ
sentry-ruby/lib/sentry/configuration.rb	98.57% <100.00%> (+0.04%)	⬆️
sentry-ruby/spec/sentry/configuration_spec.rb	100.00% <100.00%> (ø)

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[antonpirker]

The only thing I do not get is what "sending not allowed" means, but otherwise it looks good.

[st0012]

Just some suggestions on the changelog

[sl0thentr0py]

master rails is broken :( I will fix later

[st0012]

@sl0thentr0py After checking the error, it seems to be caused by Rails itself, see: rails/rails#47377. And there is already a PR for it: rails/rails#47409

So IMO we can wait a couple of days for the issue to be resolved upstream.