Best practices for native iOS Android tracing

[rtrieu]

What does this PR do? What is the motivation?

Adds a new RUM guide about best practices for tracing native iOS and Android apps.

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Preview links (active after the build_preview check completes)

New or renamed files

• https://docs-staging.datadoghq.com/rtrieu/docs-9023-native-ios-android-tracing-use-cases/real_user_monitoring/guide/best-practices-tracing-native-ios-android-apps

Modified Files

• https://docs-staging.datadoghq.com/rtrieu/docs-9023-native-ios-android-tracing-use-cases/real_user_monitoring/guide/

[MaelNamNam]

A minor change + a doubt to clarify. Otherwise LGTM. Thanks!

[MaelNamNam]

Suggested change

- **Local span sampling** applies to manually instrumented spans (like business spans or performance profiling spans). It's controlled by the `Trace.sampleRate` parameter. For example, if you set this rate to 50, all manually created spans are sent to Datadog, but only 50% are visible in the UI. Each span event includes the `_dd.agent_psr` field (the sampling rate) and `metrics._sampling_priority_v1` (1 for sampled, 0 for not sampled).

- **Local span sampling** applies to manually instrumented spans (like performance profiling spans). It's controlled by the `Trace.sampleRate` parameter. For example, if you set this rate to 50, all manually created spans are sent to Datadog, but only 50% are visible in the UI. Each span event includes the `_dd.agent_psr` field (the sampling rate) and `metrics._sampling_priority_v1` (1 for sampled, 0 for not sampled).

[MaelNamNam]

@plousada about

For example, if you set this rate to 50, all manually created spans are sent to Datadog, but only 50% are visible in the UI.

Isn't it rather

For example, if you set this rate to 50, the Trace SDK will produce spans in 50% of the cases, and send all of those to Datadog. Visibility of those spans in the UI will depend on your APM retention filters.

?