[monitor-opentelemetry] Update Monitor README (#26780)

### Packages impacted by this PR

- @azure/monitor-opentelemetry

### Issues associated with this PR

- #26754

### Describe the problem that is addressed by this PR

Updates the README with a cast to the proper type.

### What are the possible designs available to address the problem? If
there are more than one possible design, why was the one in this PR
chosen?


### Are there test cases added in this PR? _(If not, why?)_


### Provide a list of related PRs _(if any)_


### Command used to generate this PR:**_(Applicable only to SDK release
request PRs)_

### Checklists
- [x] Added impacted package name to the issue description
- [ ] Does this PR needs any fixes in the SDK Generator?** _(If so,
create an Issue in the
[Autorest/typescript](https://github.com/Azure/autorest.typescript)
repository and link it here)_
- [ ] Added a changelog (if necessary)

sdk/monitor/monitor-opentelemetry/README.md

@@ -214,26 +214,28 @@ Use a custom processor:
 ```typescript
 const { AzureMonitorOpenTelemetryClient } = require("@azure/monitor-opentelemetry");
 const { ReadableSpan, Span, SpanProcessor } = require("@opentelemetry/sdk-trace-base");
+const { NodeTracerProvider } = require("@opentelemetry/sdk-trace-node");
 const { SemanticAttributes } = require("@opentelemetry/semantic-conventions");
 
 const azureMonitorClient = new AzureMonitorOpenTelemetryClient();
 
 class SpanEnrichingProcessor implements SpanProcessor{
-    forceFlush(): Promise<void>{
-        return Promise.resolve();
-    }
-    shutdown(): Promise<void>{
-        return Promise.resolve();
-    }
-    onStart(_span: Span): void{}
-    onEnd(span: ReadableSpan){
-        span.attributes["CustomDimension1"] = "value1";
-        span.attributes["CustomDimension2"] = "value2";
-        span.attributes[SemanticAttributes.HTTP_CLIENT_IP] = "<IP Address>";
-    }
+  forceFlush(): Promise<void>{
+    return Promise.resolve();
+  }
+  shutdown(): Promise<void>{
+    return Promise.resolve();
+  }
+  onStart(_span: Span): void{}
+  onEnd(span: ReadableSpan){
+    span.attributes["CustomDimension1"] = "value1";
+    span.attributes["CustomDimension2"] = "value2";
+    span.attributes[SemanticAttributes.HTTP_CLIENT_IP] = "<IP Address>";
+  }
 }
 
-azureMonitorClient.getTracerProvider().addSpanProcessor(new SpanEnrichingProcessor());
+const tracerProvider = azureMonitorClient.getTracerProvider() as NodeTracerProvider;
+tracerProvider.addSpanProcessor(new SpanEnrichingProcessor());
 ```
 
 ### Filter telemetry