Add env var equivalent to the WithGlobal InstrumentationOption (open-…

…telemetry#849)

* Add env var equivalent to the WithGlobal InstrumentationOption

* Changelog

CHANGELOG.md

@@ -20,6 +20,7 @@ OpenTelemetry Go Automatic Instrumentation adheres to [Semantic Versioning](http
 - Support `google.golang.org/grpc` `1.65.0-dev`. ([#827](https://github.com/open-telemetry/opentelemetry-go-instrumentation/pull/827))
 - Support `google.golang.org/grpc` `1.64.0`. ([#843](https://github.com/open-telemetry/opentelemetry-go-instrumentation/pull/843))
 - `WithLoadedIndicator` `InstrumentationOption` to configure an Instrumentation to notify the caller once all the eBPF probes are loaded. ([#848](https://github.com/open-telemetry/opentelemetry-go-instrumentation/pull/848))
+- Add env var equivalent to the WithGlobal InstrumentationOption. ([#849](https://github.com/open-telemetry/opentelemetry-go-instrumentation/pull/849))
 - Support `go.opentelemetry.io/otel@v1.27.0`. ([#850](https://github.com/open-telemetry/opentelemetry-go-instrumentation/pull/850))
 
 ### Fixed

instrumentation.go

@@ -23,6 +23,7 @@ import (
 	"os"
 	"path/filepath"
 	"runtime"
+	"strconv"
 	"strings"
 
 	"github.com/go-logr/logr"
@@ -54,6 +55,9 @@ const (
 	// envTracesExportersKey is the key for the environment variable value
 	// containing what OpenTelemetry trace exporter to use.
 	envTracesExportersKey = "OTEL_TRACES_EXPORTER"
+	// envOtelGlobalImplKey is the key for the environment variable value enabling to opt-in for the
+	// OpenTelemetry global implementation. It should be a boolean value.
+	envOtelGlobalImplKey = "OTEL_GO_AUTO_GLOBAL"
 )
 
 // Instrumentation manages and controls all OpenTelemetry Go
@@ -342,9 +346,10 @@ var lookupEnv = os.LookupEnv
 //   - OTEL_GO_AUTO_TARGET_EXE: sets the target binary
 //   - OTEL_SERVICE_NAME (or OTEL_RESOURCE_ATTRIBUTES): sets the service name
 //   - OTEL_TRACES_EXPORTER: sets the trace exporter
+//   - OTEL_GO_AUTO_GLOBAL: enables the OpenTelemetry global implementation
 //
 // This option may conflict with [WithTarget], [WithPID], [WithTraceExporter],
-// and [WithServiceName] if their respective environment variable is defined.
+// [WithServiceName] and [WithGlobal] if their respective environment variable is defined.
 // If more than one of these options are used, the last one provided to an
 // [Instrumentation] will be used.
 //
@@ -372,6 +377,12 @@ func WithEnv() InstrumentationOption {
 			c.serviceName = name
 			c.additionalResAttrs = append(c.additionalResAttrs, attrs...)
 		}
+		if val, ok := lookupEnv(envOtelGlobalImplKey); ok {
+			boolVal, err := strconv.ParseBool(val)
+			if err == nil {
+				c.globalImpl = boolVal
+			}
+		}
 		return c, err
 	})
 }
@@ -448,6 +459,10 @@ func WithSampler(sampler trace.Sampler) InstrumentationOption {
 // The OpenTelemetry default global implementation is left unchanged (i.e. it
 // remains a no-op implementation) if this options is not used.
 //
+// If OTEL_GO_AUTO_GLOBAL is defined, this option will conflict with
+// [WithEnv]. If both are used, the last one provided to an [Instrumentation]
+// will be used.
+//
 // [OpenTelemetry default global implementation]: https://pkg.go.dev/go.opentelemetry.io/otel
 func WithGlobal() InstrumentationOption {
 	return fnOpt(func(_ context.Context, c instConfig) (instConfig, error) {