Metrics: add ext/aux meters

[naltatis]

refs #29144

Adds per-meter metrics.Collector for ext and aux meters, mirroring the grid/battery/pv pattern.

Todo

• manual testing @naltatis

[sourcery-ai]

Hey - I've found 1 issue, and left some high level feedback:

• The extEnergy and auxEnergy maps are populated using ref in Boot, but addMeterEnergy looks up collectors by dev.Config().Name, which can diverge from ref; consider using the same key (ref or Config().Ref) consistently for both map population and lookup.
• In addMeterEnergy, the function assumes meters and mm have matching lengths when indexing by i; if this ever changes upstream, a bounds panic is possible, so it may be safer to guard on the minimum length or explicitly validate the sizes.

Prompt for AI Agents

Please address the comments from this code review:

## Overall Comments
- The `extEnergy` and `auxEnergy` maps are populated using `ref` in `Boot`, but `addMeterEnergy` looks up collectors by `dev.Config().Name`, which can diverge from `ref`; consider using the same key (`ref` or `Config().Ref`) consistently for both map population and lookup.
- In `addMeterEnergy`, the function assumes `meters` and `mm` have matching lengths when indexing by `i`; if this ever changes upstream, a bounds panic is possible, so it may be safer to guard on the minimum length or explicitly validate the sizes.

## Individual Comments

### Comment 1
<location path="core/site.go" line_range="739" />
<code_context>
+// addMeterEnergy persists per-meter energy (positive power = import).
+func (site *Site) addMeterEnergy(key string, meters []config.Device[api.Meter], collectors map[string]*metrics.Collector, mm []types.Measurement) {
+	for i, dev := range meters {
+		c, ok := collectors[dev.Config().Name]
+		if !ok {
+			continue
</code_context>
<issue_to_address>
**issue (bug_risk):** Collector map is keyed by meter ref, but lookup uses `Config().Name`, which may not match.

`extEnergy`/`auxEnergy` are populated as `site.extEnergy[ref] = me` / `site.auxEnergy[ref] = me`, with the maps documented as keyed by meter ref. Here, the lookup uses `collectors[dev.Config().Name]`. If `ref` ≠ `Config().Name`, the collector won’t be found and energy won’t be persisted. Please align the lookup key with the map key (e.g. use `ref` or whatever is used when populating the map).
</issue_to_address>

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[naltatis]

Collected values look fine.

[andig]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Merge conflicts are resolved in 87629c2 by merging origin/master and reconciling core/site.go and core/metrics/collector.go.

Warning

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I tried to connect to the following addresses, but was blocked by firewall rules:

• 192.168.1.1
  • Triggering command: /tmp/go-build738721479/b1500/service.test /tmp/go-build738721479/b1500/service.test -test.testlogfile=/tmp/go-build738721479/b1500/testlog.txt -test.paniconexit0 -test.timeout=10m0s /tmp/go-build738721479/b1435/_pkg_.a 3.2/prometheus/p-errorsas 3.2/prometheus/p-ifaceassert ux-amd64/pkg/tool/linux_amd64/vet -errorsas ctor m ux-amd64/pkg/too-buildtags -o ux@v0.1.2/addr.g-errorsas ux@v0.1.2/const.-ifaceassert ux-amd64/pkg/tool/linux_amd64/vet -p bber mpile ux-amd64/pkg/too-buildtags (packet block)
• 224.0.0.251
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• api.awattar.at
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