Update module github.com/openzipkin/zipkin-go to v0.4.0

v0.4.0

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This release adds Baggage functionality:

• Baggage interface for the Zipkin Span Context Data model
• Baggage implementation using explicit field registration
• Baggage support for the bundled HTTP and gRPC middlewares

v0.3.0

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Mostly housekeeping and fixes for this release.

• feat(tracer): adds support for isNoop check on spans. (#​181)
• fix(propagation/b3): fix incorrect parsing of a single b3 header (#​190)
• chore(examples): makes examples use its own go.mod (#​191)
• bump(reporter/amqp): move to migrated and official library github.com/rabbitmq/amqp091-go (#​199)
• qa: license date updates, linter fixes, test fixes (#​203, #​204)
• updated dependencies and regenerated protobuf files (#​208)

Thanks all that participated including but not limited to @​jcchavezs, @​nosan, and @​SteelPhase.