[META 545] Drop unsampled transactions when connected to APM Server 8.0+

[AlexanderWert]

Drop unsampled transactions when the agent is connected to an APM Server of version 8.0 or higher.

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• Does APM server back to the supported version 6.6 have this "version" in the JSON response body of "GET /"?

[trentm]

FYI: The java agent PR: https://github.com/elastic/apm-agent-java/pull/2329/files
If I read that correctly, it doesn't block processing on having gotten a first version value from the APM server... instead it defaults to saying "yes, this is possibly an older APM server that should recent unsampled transactions" up until it has a value.

https://github.com/elastic/apm-agent-java/blob/8e15859b583fab19c12aeef25be2fc8e11227d62/apm-agent-core/src/main/java/co/elastic/apm/agent/report/ApmServerClient.java might be useful for comparison.

[trentm]

The Python agent seems to do possibly the opposite:

• It will "GET /" from APM server lazily when the first batch of events is ready to send to APM Server (setting client.server_version). If there is some error the client.server_version is left None.
• Subsequent checks whether the server is >= 8.0 will default to passing if server_version couldn't be set, or hasn't yet been set.

[trentm]

Spec ref: https://github.com/elastic/apm/blob/main/specs/agents/tracing-sampling.md#non-sampled-transactions