Add How to write MetricExporter (#2842)

docs/metrics/extending-the-sdk/MyExporter.cs

@@ -35,14 +35,23 @@ internal class MyExporter : BaseExporter<Metric>
         using var scope = SuppressInstrumentationScope.Begin();
 
         var sb = new StringBuilder();
-        foreach (var record in batch)
+        foreach (var metric in batch)
         {
             if (sb.Length > 0)
             {
                 sb.Append(", ");
             }
 
-            sb.Append($"{record}");
+            sb.Append($"{metric.Name}");
+
+            foreach (ref readonly var metricPoint in metric.GetMetricPoints())
+            {
+                sb.Append($"{metricPoint.StartTime}");
+                foreach (var metricPointTag in metricPoint.Tags)
+                {
+                    sb.Append($"{metricPointTag.Key} {metricPointTag.Value}");
+                }
+            }
         }
 
         Console.WriteLine($"{this.name}.Export([{sb}])");

docs/metrics/extending-the-sdk/README.md

@@ -7,14 +7,68 @@
 
 ## Exporter
 
-TBD
+OpenTelemetry .NET SDK has provided the following built-in metric exporters:
+
+* [Console](../../../src/OpenTelemetry.Exporter.Console/README.md)
+* [InMemory](../../../src/OpenTelemetry.Exporter.InMemory/README.md)
+* [OpenTelemetryProtocol](../../../src/OpenTelemetry.Exporter.OpenTelemetryProtocol/README.md)
+* [Prometheus](../../../src/OpenTelemetry.Exporter.Prometheus/README.md)
+
+Custom exporters can be implemented to send telemetry data to places which are
+not covered by the built-in exporters:
+
+* Exporters should derive from `OpenTelemetry.BaseExporter<Metric>` (which
+  belongs to the [OpenTelemetry](../../../src/OpenTelemetry/README.md) package)
+  and implement the `Export` method.
+* Exporters can optionally implement the `OnShutdown` method.
+* Exporters should not throw exceptions from `Export` and
+  `OnShutdown`.
+* Exporters are responsible for any retry logic needed by the scenario. The SDK
+  does not implement any retry logic.
+* Exporters should avoid generating telemetry and causing live-loop, this can be
+  done via `OpenTelemetry.SuppressInstrumentationScope`.
+* Exporters receives a batch of `Metric`, and each `Metric`
+  can contain 1 or more `MetricPoint`s.
+* Exporters should use `Activity.TagObjects` collection instead of
+  `Activity.Tags` to obtain the full set of attributes (tags).
+* Exporters should use `ParentProvider.GetResource()` to get the `Resource`
+  associated with the provider.
+
+```csharp
+class MyExporter : BaseExporter<Metric>
+{
+    public override ExportResult Export(in Batch<Metric> batch)
+    {
+        using var scope = SuppressInstrumentationScope.Begin();
+
+        foreach (var metric in batch)
+        {
+            Console.WriteLine($"Export: {metric.metric}");
+
+            foreach (ref readonly var metricPoint in metric.GetMetricPoints())
+            {
+                Console.WriteLine($"Export: {metricPoint.StartTime}");
+            }
+        }
+
+        return ExportResult.Success;
+    }
+}
+```
+
+A demo exporter which simply writes metric name and metric point start time
+, tags to the console is shown [here](./MyExporter.cs).
+
+Apart from the exporter itself, you should also provide extension methods as
+shown [here](./MyExporterExtensions.cs). This allows users to add the Exporter
+to the `MeterProvider` as shown in the example [here](./Program.cs).
 
 ## Reader
 
-TBD
+Not supported.
 
 ## Exemplar
 
-TBD
+Not supported.
 
 ## References