Add support for experimental options in the metrics API (#8111)

Part of https://github.com/open-telemetry/opentelemetry-go/issues/8110

Related to
https://github.com/open-telemetry/opentelemetry-go/issues/5882.

I'm hoping to find a better way to support experimental Options types in
our API packages. This is one approach to consider.

This contains no public API changes. It introduces a type:
`ExperimentalOption` in `/metric/internal/x`, which can be used by our
experimental options defined outside of the module. Options that embed
this interface are ignored by `New*Config` builder functions in the
metrics API to prevent them from panicing when used. Only SDKs that
explicitly support the experimental option in question will respect it.
Alternative SDKs will ignore the experimental options. We would still
need to treat ExperimentalOption as a stable artifact, since the SDK
will indirectly depend on it.

See
https://github.com/open-telemetry/opentelemetry-go/compare/main...dashpole:opentelemetry-go:attributes_advisory
for how this would be used to support the advisory attributes parameter.

trace/config.go

@@ -34,10 +34,17 @@ func (t *TracerConfig) SchemaURL() string {
 	return t.schemaURL
 }
 
+type experimentalOption interface {
+	Experimental()
+}
+
 // NewTracerConfig applies all the options to a returned TracerConfig.
 func NewTracerConfig(options ...TracerOption) TracerConfig {
 	var config TracerConfig
 	for _, option := range options {
+		if _, ok := option.(experimentalOption); ok {
+			continue
+		}
 		config = option.apply(config)
 	}
 	return config
@@ -103,6 +110,9 @@ func (cfg *SpanConfig) SpanKind() SpanKind {
 func NewSpanStartConfig(options ...SpanStartOption) SpanConfig {
 	var c SpanConfig
 	for _, option := range options {
+		if _, ok := option.(experimentalOption); ok {
+			continue
+		}
 		c = option.applySpanStart(c)
 	}
 	return c
@@ -115,6 +125,9 @@ func NewSpanStartConfig(options ...SpanStartOption) SpanConfig {
 func NewSpanEndConfig(options ...SpanEndOption) SpanConfig {
 	var c SpanConfig
 	for _, option := range options {
+		if _, ok := option.(experimentalOption); ok {
+			continue
+		}
 		c = option.applySpanEnd(c)
 	}
 	return c
@@ -167,6 +180,9 @@ func (cfg *EventConfig) StackTrace() bool {
 func NewEventConfig(options ...EventOption) EventConfig {
 	var c EventConfig
 	for _, option := range options {
+		if _, ok := option.(experimentalOption); ok {
+			continue
+		}
 		c = option.applyEvent(c)
 	}
 	if c.timestamp.IsZero() {

trace/config_test.go

@@ -544,3 +544,21 @@ func TestWithInstrumentationAttributesMerge(t *testing.T) {
 			"Attributes and attribute sets should be merged together.")
 	})
 }
+
+type testExperimentalOption struct {
+	TracerOption
+	SpanStartOption
+	SpanEndOption
+	EventOption
+}
+
+func (testExperimentalOption) Experimental() {}
+
+func TestExperimentalOptionSafe(t *testing.T) {
+	var opt testExperimentalOption
+
+	assert.NotPanics(t, func() { _ = NewTracerConfig(opt) })
+	assert.NotPanics(t, func() { _ = NewSpanStartConfig(opt) })
+	assert.NotPanics(t, func() { _ = NewSpanEndConfig(opt) })
+	assert.NotPanics(t, func() { _ = NewEventConfig(opt) })
+}