Add support for SENTRY_SAMPLE_RATE environment variable

[adinauer]

Problem Statement

The Sentry SDK documentation lists SENTRY_SAMPLE_RATE as an expected environment variable that SDKs should support. This variable controls the percentage of error events sent to Sentry (0.0 to 1.0).

The Java SDK currently supports all other expected environment variables:

• ✓ SENTRY_DSN
• ✓ SENTRY_RELEASE
• ✓ SENTRY_ENVIRONMENT
• ✓ SENTRY_TRACES_SAMPLE_RATE
• ✓ SENTRY_PROFILES_SAMPLE_RATE
• ✓ SENTRY_DEBUG
• ❌ SENTRY_SAMPLE_RATE (missing)

Current Behavior

The SentryOptions class has a sampleRate property that can be set programmatically via options.setSampleRate(), but ExternalOptions.from() does not read this property from the properties provider.

Expected Behavior

Users should be able to set SENTRY_SAMPLE_RATE environment variable to configure error event sampling.

Solution

Add the following to ExternalOptions:

1. Add a sampleRate field
2. Read it in from() via propertiesProvider.getDoubleProperty("sample-rate")
3. Merge it in SentryOptions.merge()

[linear]

JAVA-301