Add new replay context to the context protocol

[cmanallen]

This can go into a separate PR, but we should add this new Context to the protocol. see for example

relay/relay-general/src/protocol/contexts/browser.rs

Lines 6 to 16 in 23c24a9

	pub struct BrowserContext {
	/// Display name of the browser application.
	pub name: Annotated<String>,
	/// Version string of the browser.
	pub version: Annotated<String>,
	/// Additional arbitrary fields for forwards compatibility.
	#[metastructure(additional_properties, retain = "true", pii = "maybe")]
	pub other: Object<Value>,
	}

Originally posted by @jjbayer in #1791 (comment)

[jan-auer]

The information in this context seems to contain metadata on sampling. This information is usually not stored in the event payload but rather in special metadata keys (either _meta or _metrics). To resolve this issue, @cmanallen please:

1. List fields and data types that are part of this context
2. Describe how these fields are used in the product

We may need to move some of these fields to other places in the payload.

[cmanallen]

@jan-auer The fields are supplied to relay as a "replay" object within the "contexts" object containing two keys with float values.

{
    "contexts": {
        "replay": {
            "error_sample_rate": f64,
            "session_sample_rate": f64
        }
        ...
    }
}

The values are not used within the product and are instead used for internal metrics calculations. @mydea knows more and can give more details on usage.

[JoshFerge]

I am running into this now in #1983 as I didn't realize we already were setting things in the context. I think that we shouldn't store these on the replay context, and need to rethink these fields. let's discuss soon.

[jjbayer]

@cmanallen is this still relevant? Closing because of age, feel free to reopen.