Add readme for custom events

[vishweshbankwar]

Following up from: https://github.com/Azure/azure-sdk-for-net/pull/45630. I will add the changelogs details post this PR so that I can link readme from there.

[azure-sdk]

API change check

APIView has identified API level changes in this PR and created following API reviews.

Azure.Monitor.OpenTelemetry.Exporter

[rajkumar-rangaraj]

We may need to mention either in the README or the API summary that this is not vendor-agnostic and is native to Azure Monitor.

[cijothomas]

We may need to mention either in the README or the API summary that this is not vendor-agnostic and is native to Azure Monitor.

Yes but not fully. There are 2 parts

1. The ilogger log itself - anyone can send an ilogger log using the right categoy and it'll be picked up as EventTelemetry by AzMon. This part is neutral, as there is only ILogger API involved, and this log shows up everything else just like any other log would.
2. The ApplicationInsightsEventLogger wrapper, which does the above more easy, and is vendor specific.

[TimothyMothra]

Can the Changelog be updated with this PR?

[github-actions[bot]]

Hi @vishweshbankwar. Thank you for your interest in helping to improve the Azure SDK experience and for your contribution. We've noticed that there hasn't been recent engagement on this pull request. If this is still an active work stream, please let us know by pushing some changes or leaving a comment. Otherwise, we'll close this out in 7 days.

[github-actions[bot]]

Hi @vishweshbankwar. Thank you for your contribution. Since there hasn't been recent engagement, we're going to close this out. Feel free to respond with a comment containing /reopen if you'd like to continue working on these changes. Please be sure to use the command to reopen or remove the no-recent-activity label; otherwise, this is likely to be closed again with the next cleanup pass.