Messaging batch-publish: make per-message tracing disableable

[lmolkova]

Based on the discussion in open-telemetry/opentelemetry-specification#4123, we should recommend instrumentation libraries to let users control per-message tracing:

• it should be possible to opt-out of per-message tracing (via sampling or tracer suppression)
• if user opts-out, we can fallback to injecting ambient context or publish span context - this should be specified.
• maybe we can call out that for single-publish scenarios per-message span SHOULD NOT be created

Currently spec leaves it up to the instrumentation library to decide whether to trace messages independently or not.