Added

• OTLP Metric exporter supports Histogram aggregation. (#1209)
• The Code struct from the go.opentelemetry.io/otel/codes package now supports JSON marshaling and unmarshaling as well as implements the Stringer interface. (#1214)
• A Baggage API to implement the OpenTelemetry specification. (#1217)

Changed

• Set default propagator to no-op propagator. (#1184)
• The HTTPSupplier, HTTPExtractor, HTTPInjector, and HTTPPropagator from the go.opentelemetry.io/otel/api/propagation package were replaced with unified TextMapCarrier and TextMapPropagator in the go.opentelemetry.io/otel package. (#1212)
• The New function from the go.opentelemetry.io/otel/api/propagation package was replaced with NewCompositeTextMapPropagator in the go.opentelemetry.io/otel package. (#1212)
• The status codes of the go.opentelemetry.io/otel/codes package have been updated to match the latest OpenTelemetry specification.
  They now are Unset, Error, and Ok.
  They no longer track the gRPC codes. (#1214)
• The StatusCode field of the SpanData struct in the go.opentelemetry.io/otel/sdk/export/trace package now uses the codes package from this package instead of the gRPC project. (#1214)
• Move the go.opentelemetry.io/otel/api/baggage package into go.opentelemetry.io/otel/propagators. (#1217)

Fixed

• Copies of data from arrays and slices passed to go.opentelemetry.io/otel/label.ArrayValue() are now used in the returned Value instead of using the mutable data itself. (#1226)

Removed

• The ExtractHTTP and InjectHTTP functions from the go.opentelemetry.io/otel/api/propagation package were removed. (#1212)
• The Propagators interface from the go.opentelemetry.io/otel/api/propagation package was removed to conform to the OpenTelemetry specification.
  The explicit TextMapPropagator type can be used in its place as this is the Propagator type the specification defines. (#1212)
• The SetAttribute method of the Span from the go.opentelemetry.io/otel/api/trace package was removed given its redundancy with the SetAttributes method. (#1216)
• The internal implementation of Baggage storage is removed in favor of using the new Baggage API functionality. (#1217)
• Remove duplicate hostname key HostHostNameKey in Resource semantic conventions. (#1219)
• Nested array/slice support has been removed. (#1226)

Raw changes made between v0.12.0 and v0.13.0

9ebca88 Pre release v0.13.0 (#1231)
5660b0b Update label.ArrayValue to store copies of 1D arrays (#1226)
577b217 Update codes to match specification (#1214)
e9c80e6 Bump github.com/openzipkin/zipkin-go from 0.2.4 to 0.2.5 in /exporters/trace/zipkin (#1224)
b0fd424 Bump github.com/itchyny/gojq from 0.11.1 to 0.11.2 in /internal/tools (#1225)
5e66340 Add Baggage API and move Baggage propagator (#1217)
de50711 feat(span): remove SetAttribute method (#1216)
6e184cd Update propagation to conform with OpenTelemetry specification (#1212)
dc79f7f Remove duplicate hostname in Resource (#1219)
423c891 Set default propagator to no-op propagator (#1204)
04297f4 Enable exporting Histogram aggregation to OTLP metric (#1209)
a69f8fb Add integration tests for span processor ordering (#1208)
3a9f5fe Test benchmarks in precommit (#1207)