More sanity checks

[rhcarvalho]

This is an umbrella issue we may extract ideas from if needed.

The sentry-sdk-benchmark tool can perform some sanity checks while running to assert certain expectations hold. If a sanity check fail it can either abort the run or generate a visible warning in the output report.

Here are a few ideas for the future, if ever:

• Check whether Sentry SDKs are sending sessions (the original focus of the tool has been tracing/transactions, so sessions was out of the initial scope)
• Transactions should have DB spans (similarly, we also expect DB spans for OpenTelemetry apps)
• Check general shape of transactions/spans
• Check that no errors were reported (when no errors are expected)