docs: Add OpenTelemetry samples

[michaelpri10]

Adds samples for the Pub/Sub Java OpenTelemetry implementation

[snippet-bot]

Here is the summary of changes.

You are about to add 2 region tags.

• samples/snippets/src/main/java/pubsub/OpenTelemetryPublisherExample.java:19, tag pubsub_publish_otel_tracing
• samples/snippets/src/main/java/pubsub/OpenTelemetrySubscriberExample.java:19, tag pubsub_subscribe_otel_tracing

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[michaelpri10]

@hongalex Samples BOM version was updated so we can submit these samples now. PTAL when you have the chance. Thanks!