Add troubleshooting for missing transactions

[stevejgordon]

As titled.

[github-actions]

🔍 Preview links for changed docs

• troubleshoot/observability/apm-agent-dotnet/apm-net-agent.md

[benironside]

Not an expert on the content, but it makes sense to my naive eyes, and I left some suggestions for clarity of the writing. Hope they are helpful!

[stevejgordon]

Thanks for the great suggestions @benironside.

[lucabelluccini]

Thanks Steve for filing this. I wonder if this could have been actually moved around most of the APM Troubleshooting guide. This can be common across all the agents under the https://www.elastic.co/docs/solutions/observability/apm/transaction-sampling
It's true that only few of the APM Agents have the Trace Continuation Strategy, but the problem is common.