Add http.client.network_timing event for HTTP client phase timings

[surbhiia]

Fixes #3385

Changes

Adds the http.client.network_timing event so HTTP client instrumentations can report per-phase network timing when the platform exposes phase boundaries (for example on mobile clients).

Model

• New registry.http.client_network_timing attribute group in model/http/registry.yaml
• New event.http.client.network_timing in model/http/events.yaml.

Documentation

• New docs/http/http-client-network-timing.md linked from docs/http/README.md.
• Generated registry section HTTP Client Network Timing Event Attributes in /docs/registry/attributes/http.md.

Changelog

• .chloggen/http-client-network-timing-event.yaml

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Prototype PR - open-telemetry/opentelemetry-java-instrumentation#15664

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• Links to the prototypes or existing instrumentations (when adding or changing conventions)

[github-actions]

This PR contains changes to area(s) that do not have an active SIG/project and will be auto-closed:

• http

Such changes may be rejected or put on hold until a new SIG/project is established.

Please refer to the Semantic Convention Areas
document to see the current active SIGs and also to learn how to kick start a new one.

[github-actions]

This PR contains changes to area(s) that do not have an active SIG/project and will be auto-closed:

• http

Such changes may be rejected or put on hold until a new SIG/project is established.

Please refer to the Semantic Convention Areas
document to see the current active SIGs and also to learn how to kick start a new one.

[surbhiia]

@pellared @trask I've updated the PR with the suggested fixes - removal of snippets about original HTTP span attribute copy and renaming to use reltime suffix instead of time. Let me know if there are any more pending items on this. Do give this an approval and I can fix the one failing check by rebasing to the latest branch before it can be merged. Thanks!