vibankwa/aspnet enrich documentation

[vishweshbankwar]

Fixes #.

1. Adding Enrich example in Readme for AspNet instrumentation.
   Issue #1450

2. Removing the older example of Enriching the activity in AspNet from Readme.

Changes

Please provide a brief description of the changes here.

For significant contributions please make sure you have completed the following items:

• CHANGELOG.md updated for non-trivial changes
• Design discussion issue #
• Changes in public API reviewed

[linux-foundation-easycla]

The committers are authorized under a signed CLA.

• ✅ Vishwesh Bankwar (c260d3d, 98f9808, 890a1cc)

[reyang]

LGTM.

[alanwest]

Having just updated the info on the Enrich option for HttpClient instrumentation in #1244, I noted that the AspNetCore instrumentation doc contains the following verbiage:

Processor, is the general extensibility point to add additional properties to any activity. The Enrich option is specific to this instrumentation, and is provided to get access to HttpRequest and HttpResponse.

See https://github.com/open-telemetry/opentelemetry-dotnet/blob/master/src/OpenTelemetry.Instrumentation.AspNetCore/README.md#enrich.

I copied this text for consistency. Maybe the same makes sense for this doc as well.

[vishweshbankwar]

@alanwest Thanks for the feedback. I added that section. Please review.