feat(telemetry): emit compile events with page/component/state breakdown

[FarhanAliRaza]

Capture per-compile telemetry (trigger, duration, page/component/state counts, plugin usage, sanitized exception) so we can see how Reflex apps are actually built. Backend-only and short-circuited compiles are excluded so the event tracks real frontend builds.

All Submissions:

• Have you followed the guidelines stated in CONTRIBUTING.md file?
• Have you checked to ensure there aren't any other open Pull Requests for the desired changed?

Type of change

Please delete options that are not relevant.

• New feature (non-breaking change which adds functionality)

New Feature Submission:

• Does your submission pass the tests?
• Have you linted your code locally prior to submission?

Changes To Core Features:

• Have you added an explanation of what your changes do and why you'd like us to include them?
• Have you written new tests for your core changes, as applicable?
• Have you successfully ran tests with your changes locally?

[codspeed-hq]

Merging this PR will not alter performance

✅ 24 untouched benchmarks
⏩ 2 skipped benchmarks¹

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_{Comparing FarhanAliRaza:compile-events (3f6b004) with main (c3c720f)²}

Footnotes

1. 2 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

2. No successful run was found on main (39c74cb) during the generation of this report, so c3c720f was used instead as the comparison base. There might be some changes unrelated to this pull request in this report. ↩

[greptile-apps]

Greptile Summary

This PR adds a compile telemetry event to PostHog that captures per-build metadata (trigger, duration, page/component/state counts, plugin usage, and sanitized exception class). Two new modules handle the context (telemetry_context.py) and payload assembly (telemetry_accounting.py), and compiler.compile_app is changed from -> None to -> bool so backend-only short-circuit paths can suppress the event.

• TelemetryContext is a frozen dataclass that uses a ContextVar to bind the active compile context; set_exception and mutable features_used are intentionally accessible despite frozen=True.
• _prepare_event in telemetry.py is also fixed to shallow-copy the cached default properties before merging kwargs, eliminating a pre-existing mutation bug.
• features_used is wired into the payload schema and tested, but no code in this PR actually writes to it during compilation, so every emitted event will report \"features_used\": {}.

Confidence Score: 4/5

Safe to merge; telemetry errors are fully swallowed so the new code cannot affect compilation outcomes.

The change is well-structured and well-tested. The main gap is that features_used is defined, exposed in the schema, and tested with manual population, but nothing in this PR actually writes to it during a real compile. Additionally, hot-reload recompilations will emit trigger:null since no hot_reload variant exists in CompileTrigger.

reflex/utils/telemetry_context.py (missing hot_reload trigger variant, missing unit comment) and reflex/utils/telemetry_accounting.py (features_used always empty).

Important Files Changed

Filename	Overview
reflex/utils/telemetry_context.py	New frozen dataclass wrapping a ContextVar-based compile context; set_exception correctly bypasses frozen enforcement via object.setattr; elapsed_ms missing a unit comment per team convention; CompileTrigger missing a hot_reload variant.
reflex/utils/telemetry_accounting.py	New module building the compile-event payload; record_compile correctly swallows errors so telemetry never breaks builds; features_used collection is wired up but nothing in this PR populates the dict, so it will always be empty.
reflex/app.py	_compile now branches on TelemetryContext.start(); exception path correctly sets did_real_compile=True before re-raising; telemetry disabled path preserves existing behavior.
reflex/compiler/compiler.py	compile_app return type changed from None to bool; False for short-circuit paths, True for real frontend compiles; straightforward and correct.
reflex/utils/telemetry.py	Adds a properties kwarg to _prepare_event/_send/send and fixes a pre-existing mutation bug where kwargs were written directly into the cached event_data properties dict; now makes a shallow copy first.
reflex/utils/prerequisites.py	trigger parameter threaded through get_compiled_app and compile_or_validate_app; mechanical plumbing, no logic changes.
tests/units/test_app.py	Four new tests covering telemetry enabled/disabled, exception capture, and short-circuit suppression; well-structured and cover the key branches.
tests/units/utils/test_telemetry_accounting.py	Comprehensive unit tests for all accounting helpers; notably tests exception sanitization, depth hierarchy, and snapshot immunity of features_used.
tests/units/utils/test_telemetry_context.py	Tests nested context entry/exit, identity equality required by BaseContext, get() returning None before entry, and set_exception on frozen dataclass; thorough coverage.

_{Reviews (1): Last reviewed commit: "feat(telemetry): emit compile events wit..." | Re-trigger Greptile}