Make it easier to use a custom provider

Add a NewMetricsOnly provider constructor, so a custom provider can
be composed of this provider that provides access to the span based
metrics.

Add a GlobalFieldsAnnotator to expose the global fields mechanism.

Author: danmux

config/o11y/otel.go

@@ -75,6 +75,9 @@ type OtelConfig struct {
 
 	// SpanExporters allows you explicitly provide a set of exporters, as an advanced use-case.
 	SpanExporters []sdktrace.SpanExporter
+
+	// ProviderFunc is used to provide a custom provider.
+	ProviderFunc func(conf otel.Config) (o11y.Provider, error)
 }
 
 // Otel is the primary entrypoint to initialize the o11y system for otel.
@@ -90,7 +93,10 @@ func Otel(ctx context.Context, o OtelConfig) (context.Context, func(context.Cont
 	}
 	cfg.Metrics = mProv
 
-	o11yProvider, err := otel.New(cfg)
+	if o.ProviderFunc == nil {
+		o.ProviderFunc = otel.New
+	}
+	o11yProvider, err := o.ProviderFunc(cfg)
 	if err != nil {
 		return ctx, nil, err
 	}

o11y/otel/custom.go

@@ -9,6 +9,14 @@ import (
 
 var globalFields = Annotator{}
 
+// GlobalFieldsAnnotator exposes the global fields annotator. When used in conjunction with a Provider, such as the
+// MetricsOnly Provider a custom provider can implement the global fields behaviour.
+func GlobalFieldsAnnotator() *Annotator {
+	return &globalFields
+}
+
+var _ sdktrace.SpanProcessor = &Annotator{}
+
 // Annotator is a SpanProcessor that adds attributes to all started spans.
 type Annotator struct {
 	attrs []attribute.KeyValue

o11y/otel/otel.go

@@ -22,6 +22,7 @@ import (
 	sdktrace "go.opentelemetry.io/otel/sdk/trace"
 	semconv "go.opentelemetry.io/otel/semconv/v1.12.0"
 	"go.opentelemetry.io/otel/trace"
+	"go.opentelemetry.io/otel/trace/noop"
 
 	"github.com/circleci/ex/config/secret"
 	"github.com/circleci/ex/o11y"
@@ -121,6 +122,15 @@ func New(conf Config) (o11y.Provider, error) {
 	}, nil
 }
 
+// NewMetricsOnly returns a metrics only provider, to capture the span metrics behavior.
+// This can be used to compose with a custom provider to access span based metrics.
+func NewMetricsOnly(metrics o11y.ClosableMetricsProvider) o11y.Provider {
+	return &Provider{
+		metricsProvider: metrics,
+		tracer:          noop.NewTracerProvider().Tracer(""),
+	}
+}
+
 func NewHttpExporter(conf Config) (*otlptrace.Exporter, error) {
 	var serviceName, serviceVersion string
 	for _, a := range conf.ResourceAttributes {
@@ -326,7 +336,9 @@ func (o Provider) Log(ctx context.Context, name string, fields ...o11y.Pair) {
 
 func (o Provider) Close(ctx context.Context) {
 	// TODO Handle these errors in a sensible manner where possible
-	_ = o.tp.Shutdown(ctx)
+	if o.tp != nil {
+		_ = o.tp.Shutdown(ctx)
+	}
 	if o.metricsProvider != nil {
 		_ = o.metricsProvider.Close()
 	}