Do not inline noopSpan.tracerProvider (#6456)

Currently, the `noopSpan.tracerProvider` is inlined into
`noopSpan.TracerProvider` for binaries built with standard tooling. This
function is needed in the ELF file for auto-instrumentation. Ensure
this, and the other required methods are always kept isolated.

cc @open-telemetry/go-instrumentation-approvers

---------

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

CHANGELOG.md

@@ -22,6 +22,7 @@ This project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0.htm
 ### Fixes
 
 - Stop percent encoding header environment variables in `go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploggrpc` and `go.opentelemetry.io/otel/exporters/otlp/otlplog/otlploghttp`. (#6392)
+- Ensure the `noopSpan.tracerProvider` method is not inlined in `go.opentelemetry.io/otel/trace` so the `go.opentelemetry.io/auto` instrumentation can instrument non-recording spans. (#6456)
 
 <!-- Released section -->
 <!-- Don't change this section unless doing release -->

internal/global/trace.go

@@ -158,6 +158,12 @@ func (t *tracer) Start(ctx context.Context, name string, opts ...trace.SpanStart
 // a nonRecordingSpan by default.
 var autoInstEnabled = new(bool)
 
+// newSpan is called by tracer.Start so auto-instrumentation can attach an eBPF
+// uprobe to this code.
+//
+// "noinline" pragma prevents the method from ever being inlined.
+//
+//go:noinline
 func (t *tracer) newSpan(ctx context.Context, autoSpan *bool, name string, opts []trace.SpanStartOption) (context.Context, trace.Span) {
 	// autoInstEnabled is passed to newSpan via the autoSpan parameter. This is
 	// so the auto-instrumentation can define a uprobe for (*t).newSpan and be

trace/noop.go

@@ -95,6 +95,8 @@ var autoInstEnabled = new(bool)
 // tracerProvider return a noopTracerProvider if autoEnabled is false,
 // otherwise it will return a TracerProvider from the sdk package used in
 // auto-instrumentation.
+//
+//go:noinline
 func (noopSpan) tracerProvider(autoEnabled *bool) TracerProvider {
 	if *autoEnabled {
 		return newAutoTracerProvider()