Add copyright header to generated semconv packages (#7248)

Author: MrAlias

semconv/templates/registry/go/metric.go.j2

@@ -2,6 +2,9 @@
 {% import 'instrument.j2' as i -%}
 // Code generated from semantic convention specification. DO NOT EDIT.
 
+// Copyright The OpenTelemetry Authors
+// SPDX-License-Identifier: Apache-2.0
+
 // Package httpconv provides types and functionality for OpenTelemetry semantic
 // conventions in the "{{ ctx.root_namespace }}" namespace.
 package {{ ctx.root_namespace | camel_case | lower }}conv