Suggestion for making provider config optional

[c24t]

Suggestion for making config and conftest files optional in open-telemetry#466.

This tries to load third party meter providers first, then falls back to the default SDK, then uses the one that ships with the API package. It does mean the API has some knowledge of the SDK package, but only its name.

I like this solution because it doesn't require any config for common use cases. This is similar to the ServiceLoader-based approach in java.

% python -c "from opentelemetry import metrics; print(metrics.get_meter_provider().get_meter(__name__))"
<opentelemetry.sdk.metrics.Meter object at 0x108baa9d0>

% pip uninstall -y opentelemetry-sdk Found existing installation: opentelemetry-sdk 0.5.dev0
Uninstalling opentelemetry-sdk-0.5.dev0:
  Successfully uninstalled opentelemetry-sdk-0.5.dev0

% python -c "from opentelemetry import metrics; print(metrics.get_meter_provider().get_meter(__name__))"
<opentelemetry.metrics.DefaultMeter object at 0x1066b7810>