Add docs for Gauge instrument (#2276)

OpenTelemetry.sln

@@ -214,7 +214,9 @@ Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "OpenTelemetry.Instrumentati
 EndProject
 Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "OpenTelemetry.Instrumentation.AspNet.TelemetryHttpModule.Tests", "test\OpenTelemetry.Instrumentation.AspNet.TelemetryHttpModule.Tests\OpenTelemetry.Instrumentation.AspNet.TelemetryHttpModule.Tests.csproj", "{4D7201BC-7124-4401-AD65-FAB58A053D45}"
 EndProject
-Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "getting-started-histogram", "docs\metrics\getting-started-histogram\getting-started-histogram.csproj", "{92ED77A6-37B4-447D-B4C4-15DB005A589C}"
+Project("{9A19103F-16F7-4668-BE54-9A1E7A4F7556}") = "getting-started-histogram", "docs\metrics\getting-started-histogram\getting-started-histogram.csproj", "{92ED77A6-37B4-447D-B4C4-15DB005A589C}"
+EndProject
+Project("{FAE04EC0-301F-11D3-BF4B-00C04F79EFBC}") = "getting-started-async-gauge", "docs\metrics\getting-started-gauge\getting-started-async-gauge.csproj", "{E7F491CC-C37E-4A56-9CA7-8F77F59E0614}"
 EndProject
 Global
 	GlobalSection(SolutionConfigurationPlatforms) = preSolution
@@ -430,6 +432,10 @@ Global
 		{92ED77A6-37B4-447D-B4C4-15DB005A589C}.Debug|Any CPU.Build.0 = Debug|Any CPU
 		{92ED77A6-37B4-447D-B4C4-15DB005A589C}.Release|Any CPU.ActiveCfg = Release|Any CPU
 		{92ED77A6-37B4-447D-B4C4-15DB005A589C}.Release|Any CPU.Build.0 = Release|Any CPU
+		{E7F491CC-C37E-4A56-9CA7-8F77F59E0614}.Debug|Any CPU.ActiveCfg = Debug|Any CPU
+		{E7F491CC-C37E-4A56-9CA7-8F77F59E0614}.Debug|Any CPU.Build.0 = Debug|Any CPU
+		{E7F491CC-C37E-4A56-9CA7-8F77F59E0614}.Release|Any CPU.ActiveCfg = Release|Any CPU
+		{E7F491CC-C37E-4A56-9CA7-8F77F59E0614}.Release|Any CPU.Build.0 = Release|Any CPU
 	EndGlobalSection
 	GlobalSection(SolutionProperties) = preSolution
 		HideSolutionNode = FALSE
@@ -463,6 +469,7 @@ Global
 		{64E3D8BB-93AB-4571-93F7-ED8D64DFFD06} = {5B7FB835-3FFF-4BC2-99C5-A5B5FAE3C818}
 		{DFB0AD2F-11BE-4BCD-A77B-1018C3344FA8} = {3277B1C0-BDFE-4460-9B0D-D9A661FB48DB}
 		{92ED77A6-37B4-447D-B4C4-15DB005A589C} = {3277B1C0-BDFE-4460-9B0D-D9A661FB48DB}
+		{E7F491CC-C37E-4A56-9CA7-8F77F59E0614} = {3277B1C0-BDFE-4460-9B0D-D9A661FB48DB}
 	EndGlobalSection
 	GlobalSection(ExtensibilityGlobals) = postSolution
 		SolutionGuid = {55639B5C-0770-4A22-AB56-859604650521}

docs/metrics/getting-started-gauge/Program.cs

@@ -0,0 +1,52 @@
+// <copyright file="Program.cs" company="OpenTelemetry Authors">
+// Copyright The OpenTelemetry Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+// </copyright>
+
+using System;
+using System.Collections.Generic;
+using System.Diagnostics.Metrics;
+using System.Threading;
+using System.Threading.Tasks;
+using OpenTelemetry;
+using OpenTelemetry.Metrics;
+
+public class Program
+{
+    private static readonly Meter MyMeter = new Meter("TestMeter", "0.0.1");
+    private static readonly Random RandomGenerator = new Random();
+
+    public static async Task Main(string[] args)
+    {
+        using var meterProvider = Sdk.CreateMeterProviderBuilder()
+                .AddSource("TestMeter")
+                .AddConsoleExporter()
+                .Build();
+
+        ObservableGauge<long> gauge = MyMeter.CreateObservableGauge<long>(
+            "Gauge",
+            () =>
+            {
+                var tag1 = new KeyValuePair<string, object>("tag1", "value1");
+                var tag2 = new KeyValuePair<string, object>("tag2", "value2");
+
+                return new List<Measurement<long>>()
+                {
+                    new Measurement<long>(RandomGenerator.Next(1, 1000), tag1, tag2),
+                };
+            });
+
+        await Task.Delay(10000);
+    }
+}

docs/metrics/getting-started-gauge/README.md

@@ -0,0 +1,61 @@
+# Getting Started with OpenTelemetry .NET in 5 Minutes
+
+First, download and install the [.NET Core
+SDK](https://dotnet.microsoft.com/download) on your computer.
+
+Create a new console application and run it:
+
+```sh
+dotnet new console --output getting-started-async-gauge
+cd getting-started-async-gauge
+dotnet run
+```
+
+You should see the following output:
+
+```text
+Hello World!
+```
+
+Install the
+[OpenTelemetry.Exporter.Console](../../../src/OpenTelemetry.Exporter.Console/README.md)
+package:
+
+```sh
+dotnet add package OpenTelemetry.Exporter.Console
+```
+
+Update the `Program.cs` file with the code from [Program.cs](./Program.cs):
+
+Run the application again (using `dotnet run`) and you should see the metric
+output from the console, similar to shown below:
+
+<!-- markdownlint-disable MD013 -->
+```text
+Service.Nameunknown_service:getting-started-gauge
+Export 15:44:05.262 15:44:05.263 Gauge [tag1=value1;tag2=value2] LongGauge, Meter: TestMeter/0.0.1
+Value: 306
+Export 15:44:05.262 15:44:06.290 Gauge [tag1=value1;tag2=value2] LongGauge, Meter: TestMeter/0.0.1
+Value: 693
+Export 15:44:05.262 15:44:07.302 Gauge [tag1=value1;tag2=value2] LongGauge, Meter: TestMeter/0.0.1
+Value: 78
+```
+<!-- markdownlint-enable MD013 -->
+
+Congratulations! You are now collecting metrics using OpenTelemetry.
+
+What does the above program do?
+
+The program creates a
+[Meter](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#meter)
+instance named "TestMeter" and then creates a
+[Asynchronous Gauge](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#asynchronous-gauge)
+instrument from it. This Gauge reports a randomnly generated number as its
+measurement until exited after 10 seconds.
+
+An OpenTelemetry
+[MeterProvider](https://github.com/open-telemetry/opentelemetry-specification/blob/main/specification/metrics/api.md#meterprovider)
+is configured to subscribe to instruments from the Meter `TestMeter`, and
+aggregate the measurements in-memory. The pre-aggregated metrics are exported
+every 1 second to a `ConsoleExporter`. `ConsoleExporter` simply displays it on
+the console.

docs/metrics/getting-started-gauge/getting-started-async-gauge.csproj

@@ -0,0 +1,6 @@
+<Project Sdk="Microsoft.NET.Sdk">
+  <ItemGroup>
+    <ProjectReference Include="$(RepoRoot)\src\OpenTelemetry\OpenTelemetry.csproj" />
+    <ProjectReference Include="$(RepoRoot)\src\OpenTelemetry.Exporter.Console\OpenTelemetry.Exporter.Console.csproj" />
+  </ItemGroup>
+</Project>

docs/metrics/getting-started-histogram/README.md

@@ -7,7 +7,7 @@ Create a new console application and run it:
 
 ```sh
 dotnet new console --output getting-started-histogram
-cd getting-started
+cd getting-started-histogram
 dotnet run
 ```
 

src/OpenTelemetry.Exporter.Console/ConsoleMetricExporter.cs

@@ -75,13 +75,13 @@ namespace OpenTelemetry.Exporter
 
                         case MetricType.LongGauge:
                             {
-                                // TODOs
+                                valueDisplay = (metric as IGaugeMetric).LastValue.Value.ToString();
                                 break;
                             }
 
                         case MetricType.DoubleGauge:
                             {
-                                // TODOs
+                                valueDisplay = (metric as IGaugeMetric).LastValue.Value.ToString();
                                 break;
                             }