Implement change tracking functionality for Sentry integration

[matthewelwell]

See details here and here.

Note: this issue was originally created to handle the implementation of the contexts interface / evaluation tracking functionality. As per the comments on this issue, we have implemented the change tracking functionality and raised a separate item for the backlog for evaluation tracking here: #5579.

[emyller]

One trait of Sentry's Context is that it comes with some known contexts, and also allow you to add custom context. From my understanding, we want to populate the "Feature flag" known context.

Sentry offers two integrations regarding feature flags:

1. Evaluation tracking. This is useful to know the specific flags evaluated individually (per-identity) in one transaction.
2. Change tracking. This marks feature flag changes as suspect when errors occur, and also brings a visualization of changes over time in the UI.

Both integrations contribute to the Feature flags context within a Sentry issue, e.g.

Note that Evaluation tracking is the source of truth in a transaction. Change tracking has little value for segment or identity overrides.

Sentry mentions a few feature flag providers in their documentation:

• LaunchDarkly
• Statsig
• Unleash
• OpenFeature (evaluation)

It also provides a generic integration alternative path that hints at the work we'll need done in order to have Flagsmith listed in Sentry docs. Each provider seems to implement both integrations above, e.g.

• Unleash (evaluation tracking)
• Unleash (change tracking)

Scope

Evaluation tracking

This is client-side. We'll need to implement the integration Sentry's interface sentry_sdk.integrations.Integration (Python, see example) and maybe something like this example (JavaScript).

Change tracking

This is server-side. We'll need to implement the integration as a webhook sender following Sentry's spec. This also demands a new integration in Flagsmith settings, very similar to the Webhook one.

Goals:

• Implement the Evaluation tracking integration.
• Implement the Change tracking integration.
• Write Flagsmith docs.
• Propose docs to the Sentry team and get our name in there too. <3

Notes:

Sentry's support for feature flags is still in beta. Maybe as such, and according to their docs, only boolean flags are supported currently.

[emyller]

@matthewelwell After extensive testing, I've concluded that the Change Tracking doesn't seem to contribute to the Contexts interface.

Change Tracking only allows for markers in the time chart of an issue, which tells us when a feature flag was created, updated (regardless of value), or deleted. See example:

Now the Evaluation Tracking integration is the piece that contributes to Contexts, further below in the Issue detail page. This only exists when evaluating flags with the OpenFeature SDK in the client side, and an issue is raised.

Can you please help deciding what to do to the present issue? The goal of it won't be achieved in my open pull request, unfortunately. Nevertheless, I will add some documentation for users to experiment Evaluation Tracking with OpenFeature.