Add support for the development attributes advisory parameter (#8135)

From [the
spec](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#instrument-advisory-parameter-attributes):

> ##### Instrument advisory parameter: `Attributes`

> **Status**: [Development](../document-status.md)

> Applies to all instrument types.

> `Attributes` (a list of [attribute
keys](../common/README.md#attribute)) is
the recommended set of attribute keys to be used for the resulting
metrics.

We already have `WithAttributes` options, so i've opted to name this
`WithDefaultAttributes` to avoid naming collisions. We could also
consider `WithAttributeKeys`, or `WithDefaultAttributeKeys`

Follows the pattern for experimental options introduced in
https://github.com/open-telemetry/opentelemetry-go/pull/8111

---------

Co-authored-by: Robert Pająk <pellared@hotmail.com>

metric/x/README.md

@@ -0,0 +1,5 @@
+# Experimental Metric Options
+
+This package contains experimental options for the OpenTelemetry metric package.
+These options are currently under development and not part of the stable API.
+They may be changed in backwards-incompatible ways, or removed entirely.

metric/x/go.mod

@@ -0,0 +1,16 @@
+module go.opentelemetry.io/otel/metric/x
+
+go 1.25.0
+
+require (
+	go.opentelemetry.io/otel v1.43.0
+	go.opentelemetry.io/otel/metric v1.43.0
+)
+
+require github.com/cespare/xxhash/v2 v2.3.0 // indirect
+
+replace go.opentelemetry.io/otel/metric => ../../metric
+
+replace go.opentelemetry.io/otel/trace => ../../trace
+
+replace go.opentelemetry.io/otel => ../..

metric/x/go.sum

@@ -0,0 +1,18 @@
+github.com/cespare/xxhash/v2 v2.3.0 h1:UL815xU9SqsFlibzuggzjXhog7bL6oX9BbNZnL2UFvs=
+github.com/cespare/xxhash/v2 v2.3.0/go.mod h1:VGX0DQ3Q6kWi7AoAeZDth3/j3BFtOZR5XLFGgcrjCOs=
+github.com/davecgh/go-spew v1.1.1 h1:vj9j/u1bqnvCEfJOwUhtlOARqs3+rkHYY13jYWTU97c=
+github.com/davecgh/go-spew v1.1.1/go.mod h1:J7Y8YcW2NihsgmVo/mv3lAwl/skON4iLHjSsI+c5H38=
+github.com/go-logr/logr v1.4.3 h1:CjnDlHq8ikf6E492q6eKboGOC0T8CDaOvkHCIg8idEI=
+github.com/go-logr/logr v1.4.3/go.mod h1:9T104GzyrTigFIr8wt5mBrctHMim0Nb2HLGrmQ40KvY=
+github.com/go-logr/stdr v1.2.2 h1:hSWxHoqTgW2S2qGc0LTAI563KZ5YKYRhT3MFKZMbjag=
+github.com/go-logr/stdr v1.2.2/go.mod h1:mMo/vtBO5dYbehREoey6XUKy/eSumjCCveDpRre4VKE=
+github.com/google/go-cmp v0.7.0 h1:wk8382ETsv4JYUZwIsn6YpYiWiBsYLSJiTsyBybVuN8=
+github.com/google/go-cmp v0.7.0/go.mod h1:pXiqmnSA92OHEEa9HXL2W4E7lf9JzCmGVUdgjX3N/iU=
+github.com/pmezard/go-difflib v1.0.0 h1:4DBwDE0NGyQoBHbLQYPwSUPoCMWR5BEzIk/f1lZbAQM=
+github.com/pmezard/go-difflib v1.0.0/go.mod h1:iKH77koFhYxTK1pcRnkKkqfTogsbg7gZNVY4sRDYZ/4=
+github.com/stretchr/testify v1.11.1 h1:7s2iGBzp5EwR7/aIZr8ao5+dra3wiQyKjjFuvgVKu7U=
+github.com/stretchr/testify v1.11.1/go.mod h1:wZwfW3scLgRK+23gO65QZefKpKQRnfz6sD981Nm4B6U=
+go.opentelemetry.io/auto/sdk v1.2.1 h1:jXsnJ4Lmnqd11kwkBV2LgLoFMZKizbCi5fNZ/ipaZ64=
+go.opentelemetry.io/auto/sdk v1.2.1/go.mod h1:KRTj+aOaElaLi+wW1kO/DZRXwkF4C5xPbEe3ZiIhN7Y=
+gopkg.in/yaml.v3 v3.0.1 h1:fxVm/GzAzEWqLHuvctI91KS9hhNmmWOoWu0XTYJS7CA=
+gopkg.in/yaml.v3 v3.0.1/go.mod h1:K4uyk7z7BCEPqu6E+C64Yfv1cQ7kz7rIZviUmN+EgEM=

metric/x/options.go

@@ -0,0 +1,29 @@
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
+// Package x contains experimental metric options.
+package x // import "go.opentelemetry.io/otel/metric/x"
+
+import (
+	"go.opentelemetry.io/otel/attribute"
+	"go.opentelemetry.io/otel/metric"
+)
+
+type defaultAttributesOption struct {
+	metric.InstrumentOption
+	keys []attribute.Key
+}
+
+// Experimental prevents the API from panicking when the option is used.
+func (defaultAttributesOption) Experimental() {}
+
+func (o defaultAttributesOption) AllowedKeys() []attribute.Key {
+	return o.keys
+}
+
+// WithDefaultAttributes returns a metric.InstrumentOption that specifies default attribute keys.
+// The implementation should treat keys that are not included in the passed keys as opt-in, and they should be filtered out by default.
+// Users of [go.opentelemetry.io/otel/sdk/metric] can enable these attributes by using the AttributeFilter of a View to include them.
+func WithDefaultAttributes(keys ...attribute.Key) metric.InstrumentOption {
+	return defaultAttributesOption{keys: keys}
+}