refactor(metering): split into subpackages + pluggable backend#62
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…uggable backend
- metering/{file,stderr,capture}/ subpackages mirror hypervisor/snapshot
pattern: interface lives in main metering package, backends are
subpackage Recorder types exposing a New ctor.
- config.MeteringConfig selects the backend ("file" default, "nop",
"stderr"); empty config keeps v0.4.x behavior.
- cmd/core/metering.go switches over conf.Metering.Backend instead of
hard-coding NewFileRecorder.
- stderr/Recorder is a new debug backend; capture/Recorder keeps the
Entries/Reset test helpers (package doc flags it as testing-only).
- file/Recorder.New no longer takes ctx (unused) and surfaces open
errors so the cmd/core factory owns the NopRecorder fallback.
- config.MeteringBackend typed enum + constants (MeteringFile/Nop/Stderr) mirror the HypervisorType pattern; cmd/core switch now uses typed cases. - MeteringConfig.Validate() rejects unknown backends at startup (hooked into config.Config.Validate) instead of warn-and-nop at first emit. - stderr.Recorder.Emit uses Write with appended newline (matches file.Recorder.Emit shape, drops fmt.Fprintln + nolint:errcheck). - Tighten godocs: stderr.Emit doc removed (impl is obvious), capture package doc collapsed to one sentence, MeteringRecorder godoc trimmed to the lazy-init contract.
Entry now implements io.WriterTo; file and stderr recorders share the same encoding path instead of each calling json.Marshal + appending \n. Wire-format changes (encoder swap, framing, compression) are now one diff in metering.go rather than scattered across backends. file.Emit collapses two log paths (marshal-error vs write-error) into one — the wrapped error already carries the cause, and downstream cares only that the entry failed.
- WriteTo: "writes one JSONL record" (was: wire-format + downstream tools). - file.Recorder / file.Emit: drop the JSON-encoding narrative now that it lives on Entry.WriteTo; keep only the file-specific WHY (O_APPEND atomicity, swallow rationale). - stderr.Recorder: routes to os.Stderr instead of restating the wire format. - config: drop v0.4.x backstory; the default-behavior sentence carries the same information. - metering_test: drop the comment that just paraphrased the test name.
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Summary
Reshape `metering/` to match cocoon's interface-in-main + backend-in-subpackage pattern (hypervisor / snapshot / images / network). Backend is now configurable via `config.MeteringConfig` instead of hard-coded in `cmd/core`.
New layout
```
config/
└── metering.go (MeteringConfig / FileMeteringConfig)
metering/
├── metering.go (Recorder interface, Entry/Shape/Kind/Reason, NopRecorder)
├── file/ (production: JSONL append on disk)
├── stderr/ (dev/debug: JSON line to stderr)
└── capture/ (testing-only: buffered with Entries/Reset)
```
Each subpackage exposes `Recorder` struct + `New` ctor + compile-time `var _ metering.Recorder = (*Recorder)(nil)`.
Config
```jsonc
// default — equivalent to v0.4.x file behavior, no config change needed
{}
// explicit
{"metering": {"backend": "file", "file": {"path": ""}}}
// disable
{"metering": {"backend": "nop"}}
// dev
{"metering": {"backend": "stderr"}}
```
Factory
`cmd/core/metering.go` switches over `conf.Metering.Backend`; unknown backend warns + falls back to NopRecorder. Adding a new backend (HTTP webhook, Pub/Sub, Kafka, …) is one new subpackage exposing a Recorder + adding one case to the switch.
Other notable changes
Backward compat
Test plan