trace,metric,log: WithInstrumentationAttributes options to merge attributes (#7300)

## What

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.

## Why

Per
https://github.com/open-telemetry/opentelemetry-go/pull/7287#pullrequestreview-3181379062
and
https://github.com/open-telemetry/opentelemetry-go/pull/7287#issuecomment-3250085450

Not that this does not address
https://github.com/open-telemetry/opentelemetry-go/issues/7217. This is
left to a seperate PR to not scope creep this one. CC @axw

---------

Co-authored-by: Damien Mathieu <42@dmathieu.com>

metric/config.go

@@ -65,9 +65,24 @@ func WithInstrumentationVersion(version string) MeterOption {
 // WithInstrumentationAttributes sets the instrumentation attributes.
 //
 // The passed attributes will be de-duplicated.
+//
+// If multiple WithInstrumentationAttributes options are passed the
+// attributes will be merged together in the order they are passed. Attributes
+// with duplicate keys will use the last value passed.
 func WithInstrumentationAttributes(attr ...attribute.KeyValue) MeterOption {
+	if len(attr) == 0 {
+		return meterOptionFunc(func(config MeterConfig) MeterConfig {
+			return config
+		})
+	}
+
 	return meterOptionFunc(func(config MeterConfig) MeterConfig {
-		config.attrs = attribute.NewSet(attr...)
+		newAttrs := attribute.NewSet(attr...)
+		if config.attrs.Len() == 0 {
+			config.attrs = newAttrs
+		} else {
+			config.attrs = mergeSets(config.attrs, newAttrs)
+		}
 		return config
 	})
 }

metric/config_test.go

@@ -30,3 +30,51 @@ func TestConfig(t *testing.T) {
 	assert.Equal(t, schemaURL, c.SchemaURL(), "schema URL")
 	assert.Equal(t, attr, c.InstrumentationAttributes(), "instrumentation attributes")
 }
+
+func TestWithInstrumentationAttributesMerge(t *testing.T) {
+	aliceAttr := attribute.String("user", "Alice")
+	bobAttr := attribute.String("user", "Bob")
+	adminAttr := attribute.Bool("admin", true)
+
+	alice := attribute.NewSet(aliceAttr)
+	bob := attribute.NewSet(bobAttr)
+	aliceAdmin := attribute.NewSet(aliceAttr, adminAttr)
+	bobAdmin := attribute.NewSet(bobAttr, adminAttr)
+
+	t.Run("SameKey", func(t *testing.T) {
+		c := metric.NewMeterConfig(
+			metric.WithInstrumentationAttributes(aliceAttr),
+			metric.WithInstrumentationAttributes(bobAttr),
+		)
+		assert.Equal(t, bob, c.InstrumentationAttributes(),
+			"Later values for the same key should overwrite earlier ones.")
+	})
+
+	t.Run("DifferentKeys", func(t *testing.T) {
+		// Different keys should be merged
+		c := metric.NewMeterConfig(
+			metric.WithInstrumentationAttributes(aliceAttr),
+			metric.WithInstrumentationAttributes(adminAttr),
+		)
+		assert.Equal(t, aliceAdmin, c.InstrumentationAttributes(),
+			"Different keys should be merged.")
+	})
+
+	t.Run("Mixed", func(t *testing.T) {
+		c := metric.NewMeterConfig(
+			metric.WithInstrumentationAttributes(aliceAttr, adminAttr),
+			metric.WithInstrumentationAttributes(bobAttr),
+		)
+		assert.Equal(t, bobAdmin, c.InstrumentationAttributes(),
+			"Combination of same and different keys should be merged.")
+	})
+
+	t.Run("MergedEmpty", func(t *testing.T) {
+		c := metric.NewMeterConfig(
+			metric.WithInstrumentationAttributes(aliceAttr),
+			metric.WithInstrumentationAttributes(),
+		)
+		assert.Equal(t, alice, c.InstrumentationAttributes(),
+			"Empty attributes should not affect existing ones.")
+	})
+}