datadog: honor DD environment variables

[toumorokoshi]

There's a few environment variables that are not honored by the exporter currently. These are:

• DD_AGENT_HOST, for the agent hostname
• DATADOG_SERVICE_NAME, DD_SERVICE_NAME, DD_SERVICE for the datadog service name.

There is a more comprehensive list, as outlined here:

https://docs.datadoghq.com/tracing/setup/python/#configuration

[toumorokoshi]

another one :DD_ENV: https://docs.datadoghq.com/tracing/setup/python/#configuration

[majorgreys]

Thanks for raising these issues @toumorokoshi.

I only added support for configuring DD_TRACE_AGENT_URL whereas setting the agent hostname and port have been deprecated (though the documentation you linked to does not reflect that, which we will address):

https://github.com/open-telemetry/opentelemetry-python/blob/master/ext/opentelemetry-ext-datadog/src/opentelemetry/ext/datadog/exporter.py#L63-L67
http://pypi.datadoghq.com/trace/docs/configuration.html

Similarly, the exporter supports DD_SERVICE whereas the DATADOG_SERVICE_NAME and DD_SERVICE_NAME have been deprecated.

I'll start a PR for adding DD_ENV, as well as the other tracer configuration options missing (DD_VERSION for adding the application version to the root span and DD_TAGS for adding default tags to all spans).