feat: allow config of before_send function to edit or reject events

[pauldambra]

see RFC: https://github.com/PostHog/product-internal/pull/668/ see docs PR: https://github.com/PostHog/posthog.com/pull/9844

The number of config options to redact urls or edit properties is growing It is simpler to have a single hook where you can edit an event before it is captured

This also allows the enterprising customer configure a sampling function

---

tested locally and event capture still works without a beforeSend set and e.g. the pre-canned event sampling function works as expected

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Name	Status	Preview	Updated (UTC)
posthog-js	✅ Ready (Inspect)	Visit Preview	Nov 19, 2024 11:36am

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Size Change: +10.4 kB (+0.33%)

Total Size: 3.12 MB

Filename	Size	Change
dist/all-external-dependencies.js	204 kB	-6 B (0%)
dist/array.full.es5.js	254 kB	+1.06 kB (+0.42%)
dist/array.full.js	357 kB	+940 B (+0.26%)
dist/array.full.no-external.js	356 kB	+940 B (+0.27%)
dist/array.js	171 kB	+943 B (+0.55%)
dist/array.no-external.js	170 kB	+941 B (+0.56%)
dist/dead-clicks-autocapture.js	13.6 kB	+78 B (+0.58%)
dist/external-scripts-loader.js	2.27 kB	+74 B (+3.37%)
dist/main.js	172 kB	+1.01 kB (+0.59%)
dist/module.full.js	357 kB	+1 kB (+0.28%)
dist/module.full.no-external.js	356 kB	+1 kB (+0.28%)
dist/module.js	171 kB	+1 kB (+0.59%)
dist/module.no-external.js	170 kB	+1 kB (+0.59%)
dist/recorder-v2.js	113 kB	+71 B (+0.06%)
dist/recorder.js	113 kB	+71 B (+0.06%)
dist/surveys-preview.js	56.7 kB	+71 B (+0.13%)
dist/surveys.js	62.1 kB	+81 B (+0.13%)
dist/tracing-headers.js	1.4 kB	+74 B (+5.57%)	🔍

ℹ️ View Unchanged

| Filename | Size | | :--- | :---: | | `dist/exception-autocapture.js` | 8.7 kB | | `dist/web-vitals.js` | 10.3 kB |

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[pauldambra]

have updated and tested the examples in the posthog.com docs PR

and tested this locally

i think this is good to go

[pauldambra]

@robbie-c how about allowing an array of functions...

then we can do

before_send: [
    sampleByDistinctId(0.5), // only half of people
    sampleByEvent(['$web_vitals], 0.1), // and they capture all events except 10% of web vitals
]

[pauldambra]

updated docs PR here https://github.com/PostHog/posthog.com/pull/9844/files#diff-e9e9ef37e4e756e5cf3bc804e89db87a8e60864b4a39d43c61c311cd0ae13eb6R583-R596

[robbie-c]

Makes sense, would you multiply the sample thresholds together?