Create basic_metric example

[hectorhdzg]

Similar to
https://github.com/open-telemetry/opentelemetry-python/tree/master/examples/basic_tracer
showing E2E flow using multiple exporters

[mauriciovasquezbernal]

I created https://github.com/open-telemetry/opentelemetry-python/tree/master/docs/examples/basic_meter as part of #483. @hectorhdzg do you think that satisfies the requirements of this issue?

[c24t]

@mauriciovasquezbernal, @hectorhdzg can we close this one?

[hectorhdzg]

I believe we have nice examples now thanks @mauriciovasquezbernal, so this could be closed now