Add an example for integration testing against OTEL collector with local file sink

[dimaqq]

From: https://github.com/canonical/operator/issues/1757 Ref: https://github.com/canonical/opentelemetry-collector-k8s-operator/issues/143

When there's a config option to deploy OTEL collector to dump trace data into a file, we should add an integration testing example in our tracing documentation.