feederv4: optional MongoDB sink for set requests

[javogc]

What

An optional output for feederv4 that stores the in-scope Set requests the feeder receives from the server in a MongoDB collection.

MONGODB_URI=<connection string> ./feederv4 --mongosink [--mongodb <database>] [--mongocoll <collection>]

The database and collection default to vissr / feederSets. Each Set request becomes one document: dot-separated paths become nested documents, and numeric/boolean values keep their native type so they can be queried and aggregated, e.g.

{"_id": "2026-10-05T23:02:44.611Z", "ts": "2026-10-05T23:02:44.611Z", "Vehicle": {"Speed": 50}, "updatedAt": <date>}

Why

This lets a cloud-side VISSR deployment persist incoming vehicle data for history, analytics or backend applications without having to write a custom feeder.

Design notes

• Backward compatible: the sink is off by default, and nothing changes when --mongosink is not used.
• Grouping: the server delivers a multi-set request to the feeder as one message per data point. Data points arriving within 200 ms of each other are grouped, share the timestamp of the first one, and are upserted into the same document.
• Non-blocking: writes go through a bounded queue on a background goroutine. A slow or unreachable database drops data points with an error log and never blocks the feeder.
• Documented in feeder/feeder-template/feederv4/README.md.

Dependencies

Adds go.mongodb.org/mongo-driver/v2 (v2.4.0). As a result go mod tidy bumps the indirect golang.org/x/{net,sync,sys,text} modules and adds golang.org/x/crypto plus a few indirect driver dependencies. The go directive stays at 1.22.1. If you'd rather keep the module free of the driver, we're happy to put the sink behind a build tag.

Testing

• Unit tests for the document mapping, typed values and request grouping (go test -race ./feeder/feeder-template/feederv4/).
• go build ./..., go vet and go test -race over the module (same as the test workflow) pass locally.
• Exercised end to end against MongoDB Atlas.

Branches

Per the README, features merged to master are also carried to v3.1.5, v3.2 and v4.0. Let us know if you'd like us to open those PRs.