## Overview

### Added

- Greatly reduce the cost of recording metrics in
`go.opentelemetry.io/otel/sdk/metric` using hashing for map keys.
(#7175)
- Add `WithInstrumentationAttributeSet` option to
`go.opentelemetry.io/otel/log`, `go.opentelemetry.io/otel/metric`, and
`go.opentelemetry.io/otel/trace` packages. This provides a
concurrent-safe and performant alternative to
`WithInstrumentationAttributes` by accepting a pre-constructed
`attribute.Set`. (#7287)
- Add experimental observability for the Prometheus exporter in
`go.opentelemetry.io/otel/exporters/prometheus`. Check the
`go.opentelemetry.io/otel/exporters/prometheus/internal/x` package
documentation for more information. (#7345)
- Add experimental observability metrics in
`go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploggrpc`. (#7353)
- Add temporality selector functions `DeltaTemporalitySelector`,
`CumulativeTemporalitySelector`, `LowMemoryTemporalitySelector` to
`go.opentelemetry.io/otel/sdk/metric`. (#7434)
- Add experimental observability metrics for simple log processor in
`go.opentelemetry.io/otel/sdk/log`. (#7548)
- Add experimental observability metrics in
`go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc`.
(#7459)
- Add experimental observability metrics in
`go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp`.
(#7486)
- Add experimental observability metrics for simple span processor in
`go.opentelemetry.io/otel/sdk/trace`. (#7374)
- Add experimental observability metrics in
`go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploghttp`. (#7512)
- Add experimental observability metrics for manual reader in
`go.opentelemetry.io/otel/sdk/metric`. (#7524)
- Add experimental observability metrics for periodic reader in
`go.opentelemetry.io/otel/sdk/metric`. (#7571)
- Support `OTEL_EXPORTER_OTLP_LOGS_INSECURE` and
`OTEL_EXPORTER_OTLP_INSECURE` environmental variables in
`go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploghttp`. (#7608)
- Add `Enabled` method to the `Processor` interface in
`go.opentelemetry.io/otel/sdk/log`. All `Processor` implementations now
include an `Enabled` method. (#7639)
- The `go.opentelemetry.io/otel/semconv/v1.38.0` package. The package
contains semantic conventions from the `v1.38.0` version of the
OpenTelemetry Semantic Conventions. See the [migration
documentation](./semconv/v1.38.0/MIGRATION.md) for information on how to
upgrade from `go.opentelemetry.io/otel/semconv/v1.37.0.`(#7648)

### Changed

- `Distinct` in `go.opentelemetry.io/otel/attribute` is no longer
guaranteed to uniquely identify an attribute set. Collisions between
`Distinct` values for different Sets are possible with extremely high
cardinality (billions of series per instrument), but are highly
unlikely. (#7175)
- `WithInstrumentationAttributes` in `go.opentelemetry.io/otel/trace`
synchronously de-duplicates the passed attributes instead of delegating
it to the returned `TracerOption`. (#7266)
- `WithInstrumentationAttributes` in `go.opentelemetry.io/otel/meter`
synchronously de-duplicates the passed attributes instead of delegating
it to the returned `MeterOption`. (#7266)
- `WithInstrumentationAttributes` in `go.opentelemetry.io/otel/log`
synchronously de-duplicates the passed attributes instead of delegating
it to the returned `LoggerOption`. (#7266)
- Rename the `OTEL_GO_X_SELF_OBSERVABILITY` environment variable to
`OTEL_GO_X_OBSERVABILITY` in `go.opentelemetry.io/otel/sdk/trace`,
`go.opentelemetry.io/otel/sdk/log`, and
`go.opentelemetry.io/otel/exporters/stdout/stdouttrace`. (#7302)
- Improve performance of histogram `Record` in
`go.opentelemetry.io/otel/sdk/metric` when min and max are disabled
using `NoMinMax`. (#7306)
- Improve error handling for dropped data during translation by using
`prometheus.NewInvalidMetric` in
`go.opentelemetry.io/otel/exporters/prometheus`. ⚠️ **Breaking Change:**
Previously, these cases were only logged and scrapes succeeded. Now,
when translation would drop data (e.g., invalid label/value), the
exporter emits a `NewInvalidMetric`, and Prometheus scrapes **fail with
HTTP 500** by default. To preserve the prior behavior (scrapes succeed
while errors are logged), configure your Prometheus HTTP handler with:
`promhttp.HandlerOpts{ ErrorHandling: promhttp.ContinueOnError }`.
(#7363)
- Replace fnv hash with xxhash in `go.opentelemetry.io/otel/attribute`
for better performance. (#7371)
- The default `TranslationStrategy` in
`go.opentelemetry.io/exporters/prometheus` is changed from
`otlptranslator.NoUTF8EscapingWithSuffixes` to
`otlptranslator.UnderscoreEscapingWithSuffixes`. (#7421)
- Improve performance of concurrent measurements in
`go.opentelemetry.io/otel/sdk/metric`. (#7427)
- Include W3C TraceFlags (bits 0–7) in the OTLP `Span.Flags` field in
`go.opentelemetry.io/exporters/otlp/otlptrace/otlptracehttp` and
`go.opentelemetry.io/exporters/otlp/otlptrace/otlptracegrpc`. (#7438)
- The `ErrorType` function in `go.opentelemetry.io/otel/semconv/v1.37.0`
now handles custom error types.
If an error implements an `ErrorType() string` method, the return value
of that method will be used as the error type. (#7442)

### Fixed

- Fix `WithInstrumentationAttributes` options in
`go.opentelemetry.io/otel/trace`, `go.opentelemetry.io/otel/metric`, and
`go.opentelemetry.io/otel/log` to properly merge attributes when passed
multiple times instead of replacing them. Attributes with duplicate keys
will use the last value passed. (#7300)
- The equality of `attribute.Set` when using the `Equal` method is not
affected by the user overriding the empty set pointed to by
`attribute.EmptySet` in `go.opentelemetry.io/otel/attribute`. (#7357)
- Return partial OTLP export errors to the caller in
`go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploggrpc`. (#7372)
- Return partial OTLP export errors to the caller in
`go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploghttp`. (#7372)
- Return partial OTLP export errors to the caller in
`go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc`.
(#7372)
- Return partial OTLP export errors to the caller in
`go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetrichttp`.
(#7372)
- Return partial OTLP export errors to the caller in
`go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracegrpc`.
(#7372)
- Return partial OTLP export errors to the caller in
`go.opentelemetry.io/otel/exporters/otlp/otlptrace/otlptracehttp`.
(#7372)
- Fix `AddAttributes`, `SetAttributes`, `SetBody` on `Record` in
`go.opentelemetry.io/otel/sdk/log` to not mutate input. (#7403)
- Do not double record measurements of `RecordSet` methods in
`go.opentelemetry.io/otel/semconv/v1.37.0`. (#7655)
- Do not double record measurements of `RecordSet` methods in
`go.opentelemetry.io/otel/semconv/v1.36.0`. (#7656)

### Removed

- Drop support for [Go 1.23]. (#7274)
- Remove the `FilterProcessor` interface in
`go.opentelemetry.io/otel/sdk/log`. The `Enabled` method has been added
to the `Processor` interface instead. All `Processor` implementations
must now implement the `Enabled` method. Custom processors that do not
filter records can implement `Enabled` to return `true`. (#7639)