Programmatic configuration

[c24t]

From @toumorokoshi's comment on #466 .

Right now it's possible to set the global tracer and meter providers in code, but not to change Configuration, which is always created from static config. Ideally we'd give users the option to set config values in code as well as in the configuration file.

cc @ocelotl

[c24t]

Closing in favor of #554