Add OpenCensus reader bridge PoC

bridge/opencensus/metric.go

@@ -22,6 +22,7 @@ import (
 
 	ocmetricdata "go.opencensus.io/metric/metricdata"
 	"go.opencensus.io/metric/metricexport"
+	"go.opencensus.io/metric/metricproducer"
 
 	internal "go.opentelemetry.io/otel/bridge/opencensus/internal/ocmetric"
 	"go.opentelemetry.io/otel/sdk/instrumentation"
@@ -38,7 +39,7 @@ type exporter struct {
 }
 
 // NewMetricExporter returns an OpenCensus exporter that exports to an
-// OpenTelemetry exporter.
+// OpenTelemetry (push) exporter.
 func NewMetricExporter(base metric.Exporter, res *resource.Resource) metricexport.Exporter {
 	return &exporter{base: base}
 }
@@ -61,3 +62,43 @@ func (e *exporter) ExportMetrics(ctx context.Context, ocmetrics []*ocmetricdata.
 			},
 		}})
 }
+
+type reader struct {
+	// This reader doesn't actually matter. It is just there to satisfy the Reader interface.
+	metric.Reader
+	res     *resource.Resource
+	manager *metricproducer.Manager
+}
+
+// NewMetricExporter returns an OpenTelemetry metric.Reader that can be used as
+// a source of metrics for OpenTelemetry (pull) exporters.
+func NewMetricReader(res *resource.Resource) metric.Reader {
+	return &reader{
+		Reader:  metric.NewManualReader(),
+		res:     res,
+		manager: metricproducer.GlobalManager(),
+	}
+}
+
+// Collect overrides the manual reader's collect method to retrieve metrics from OpenCensus.
+func (r *reader) Collect(context.Context) (metricdata.ResourceMetrics, error) {
+	producers := r.manager.GetAll()
+	data := []*ocmetricdata.Metric{}
+	for _, ocProducer := range producers {
+		data = append(data, ocProducer.Read()...)
+	}
+	otelmetrics, err := internal.ConvertMetrics(data)
+	if err != nil {
+		return metricdata.ResourceMetrics{}, err
+	}
+	return metricdata.ResourceMetrics{
+		Resource: r.res,
+		ScopeMetrics: []metricdata.ScopeMetrics{
+			{
+				Scope: instrumentation.Scope{
+					Name: "go.opentelemetry.io/otel/bridge/opencensus",
+				},
+				Metrics: otelmetrics,
+			},
+		}}, nil
+}

exporters/prometheus/exporter.go

@@ -44,22 +44,41 @@ type collector struct {
 }
 
 // config is added here to allow for options expansion in the future.
-type config struct{}
+type config struct {
+	reader metric.Reader
+}
 
 // Option may be used in the future to apply options to a Prometheus Exporter config.
 type Option interface {
 	apply(config) config
 }
 
+type optionFunc func(config) config
+
+func (fn optionFunc) apply(cfg config) config {
+	return fn(cfg)
+}
+
+// WithReader provides a Reader from which the prometheus exporter reads metrics.
+func WithReader(r metric.Reader) Option {
+	return optionFunc(func(config config) config {
+		config.reader = r
+		return config
+	})
+}
+
 // New returns a Prometheus Exporter.
-func New(_ ...Option) Exporter {
+func New(opts ...Option) Exporter {
 	// this assumes that the default temporality selector will always return cumulative.
 	// we only support cumulative temporality, so building our own reader enforces this.
-	reader := metric.NewManualReader()
+	cfg := config{reader: metric.NewManualReader()}
+	for _, o := range opts {
+		cfg = o.apply(cfg)
+	}
 	e := Exporter{
-		Reader: reader,
+		Reader: cfg.reader,
 		Collector: &collector{
-			Reader: reader,
+			Reader: cfg.reader,
 		},
 	}
 	return e