OTEL_SERVICE_NAME is not supported by default

[erichulburd]

opentelemetry-rust support OTEL_SERVICE_NAME when the resource is None; it then uses an environment detector: https://github.com/open-telemetry/opentelemetry-rust/pull/571/files

The issue here is that we are explicitly setting resource

pyo3-opentelemetry/crates/tracing-subscriber/src/layers/otel_otlp.rs

Line 103 in e84b8b3

.with_resource(self.resource.clone()),

and PyResource defaults to an empty map, rather than None:

pyo3-opentelemetry/crates/tracing-subscriber/src/layers/otel_otlp.rs

Line 304 in e84b8b3

attrs: attrs.unwrap_or_default(),