Heartbeat: add journey.duration mapping to synthetics event

[vigneshshanmugam]

• Part of [Heartbeat] Synthetics journey/end event has incorrect mapping for start/end time #40208 which adds the journey.duration.us field mapping to follow up with the change on the Synthetics library feat: measure startup delay and journey duration synthetics#709. which gets rid of start/end timestamps that causes parsing errors on the ES side. This aligns with the previous step duration field.

• My code follows the style guidelines of this project
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• I have made corresponding change to the default configuration files
• I have added tests that prove my fix is effective or that my feature works
• I have added an entry in CHANGELOG.next.asciidoc or CHANGELOG-developer.next.asciidoc.

[elasticmachine]

Pinging @elastic/obs-ds-hosted-services (Team:obs-ds-hosted-services)

[mergify]

This pull request does not have a backport label.
If this is a bug or security fix, could you label this PR @vigneshshanmugam? 🙏.
For such, you'll need to label your PR with:

• The upcoming major version of the Elastic Stack
• The upcoming minor version of the Elastic Stack (if you're not pushing a breaking change)

To fixup this pull request, you need to add the backport labels for the needed
branches, such as:

• backport-v8./d.0 is the label to automatically backport to the 8./d branch. /d is the digit

[emilioalvap]

LGTM