Migrating from tracing-stackdriver

Swap the tracing-stackdriver layer for GcpCloudLoggingLayer at the same position in the subscriber stack:

- .with(tracing_stackdriver::layer().with_cloud_trace(
-     tracing_stackdriver::CloudTraceConfiguration { project_id },
- ))
+ .with(gcloud_observability::logs::GcpCloudLoggingLayerBuilder::new(project_id).build()?)

The layer reads the trace and span IDs through the public tracing-opentelemetry API, so register tracing_opentelemetry::layer() before it, see The logging layer.

Two things change in the emitted log lines:

• there is no span or spans object carrying the enclosing span’s name and fields: the span attributes live on the trace, and are not repeated into every log line;
• event field names are written as given, not camel-cased; only the httpRequest.* keys are camel-cased, matching Google’s own field names, see Field conventions.

A layer has one sink. JSON lines on stdout, the default, fit GKE, Cloud Run and hosts with the Cloud Logging agent.