Make ActivitySource static (#67)

* Make ActivitySource static

* Switch CI workflow to ubuntu-latest

Author: stevejgordon

.github/workflows/ci.yml

@@ -16,7 +16,7 @@ on:
 
 jobs:
   build:
-    runs-on: ubuntu-18.04
+    runs-on: ubuntu-latest
     steps:
     - uses: actions/checkout@v2
       with:

src/Elastic.Transport/Components/Pipeline/DefaultRequestPipeline.cs

@@ -33,7 +33,7 @@ public class DefaultRequestPipeline<TConfiguration> : RequestPipeline
 	private readonly TConfiguration _settings;
 	private readonly ResponseBuilder _responseBuilder;
 
-	private readonly ActivitySource _activitySource = new("Elastic.Transport.RequestPipeline");
+	private static readonly ActivitySource _activitySource = new("Elastic.Transport.RequestPipeline");
 
 	private RequestConfiguration _pingAndSniffRequestConfiguration;
 	private List<Audit> _auditTrail = null;