Response.whenSent() only works when called before Response.send()

[alwaysthecritic]

Environment Details

• Helidon Version: 4.0.9
• Helidon SE
• JDK version: OpenJDK 64-Bit Server VM Temurin-21.0.3+9 (build 21.0.3+9-LTS, mixed mode)
• OS: macOS Sonoma 14.5
• Docker version (if applicable): N/A

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Problem Description

This documentation page on Metrics (https://helidon.io/docs/latest/se/guides/metrics) has a Timer code sample, with respond.send() followed by response.whenSent(), but in my code, if I do it that way around, I get no data recorded in my Timer metric. If I swap the order and call response.whenSent() first, then it works.

So either the documentation that includes such examples needs to be updated, or the code needs to be changed to work either way around. Regardless the Javadoc for the whenSent() method should probably explicitly describe what works (only before, or before/after).