metering: MetricsSender.Send reads identity from the context

[matthewdarwin]

Alternative to #979; only one of the two is needed.

Both callers of MetricsSender.Send repeat the same block: dauth.FromContext, then organization id, api key id, meta and ip, passed as four string arguments. The callers are tier1's Blocks response handler and tier2's end-of-ProcessRange in pipeline/terminate.go. tier1 also copies these values once per stream; the copy dates from the tier split in #238, when trusted headers could not change during a stream.

This PR changes Send(ctx, organizationID, apiKeyID, ip, userMeta, outputModuleHash, endpoint) to Send(ctx, outputModuleHash, endpoint). Send reads the identity from dauth.FromContext(ctx) on every call.

• Removes the duplicated identity handling from both callers.
• Every event uses the current headers. That is what grpc: refresh trusted headers from continuous auth responses dauth#19 needs, since it lets continuous authentication refresh a stream's trusted headers.
• A context without trusted headers emits empty identity fields, as callers passing empty strings did before.

Send is exported. A GitHub code search finds no callers outside this repo.

#979 makes the minimal fix: tier1 reads the headers per response and leaves Send's signature alone. This PR fixes the same thing by removing the duplication. Happy to close whichever one isn't wanted.

Tests

• TestSend puts identity on the context through dauth.WithTrustedHeaders and checks the emitted fields. TestSend_noTrustedHeaders covers the empty case.
• go test ./metering/ ./pipeline/ ./service/ ./test/ passes, and go test -race ./metering/ passes.
• go test -race ./test/ reports data races both with and without this change. They come from responseCollector.Collect appending to its slices from several pipeline goroutines (test/collector_test.go:93), not from this change.

🤖 Generated with Claude Code

[GabrielCartier]

Logic checks out: respContext derives from the auth middleware ctx and nothing re-wraps WithTrustedHeaders, so the per-call read in Send will see dauth#19's refreshed holder. Builds and tests green against both current dauth and the dauth#19 branch, merges clean onto develop.

One case neither this nor #979 covers, fine as a follow-up: LiveBackFiller.Start (service/live_back_filler.go:145) snapshots ToOutgoingGRPCContext once, and every live-backfill ProcessRange reuses it, so tier2 bills those jobs with the stream-start identity for the whole live stream. That is the hours-long case dauth#19 describes. Moving the ToOutgoingGRPCContext call into requestBackProcessing, as worker.go:348 already does per job, would close it.