docs: clarify downstream adapter validation requirements (#2101)

## Summary
- document that downstream `MultiCollector` adapters must expose
registration metadata that matches the metric families they emit at
scrape time
- make the newer suffix/collision validation model more explicit for
adapter maintainers

## Why
- the existing release notes already describe the suffix-handling and
collision-model change
- what was missing was explicit guidance that downstream adapters also
need their registration-time metadata to stay aligned with emitted
`MetricSnapshot` families

## Testing
- `mise run lint`

Author: zeitlinger

CHANGELOG.md

@@ -25,6 +25,12 @@
 > To keep the Prometheus and OM1 output unambiguous, the registry tracks claimed exposition names
 > and still rejects registrations that would collide at scrape time.
 >
+> Downstream adapter libraries that implement `MultiCollector` need their registration-time
+> metadata to match the metric families they emit at scrape time. When upgrading to 1.6.0+, adapter
+> registration metadata needs to stay aligned with emitted names, types, label names, and suffix
+> behavior under the new collision model.
+> See also: [Validation at registration only](docs/content/getting-started/registry.md#validation-at-registration-only)
+>
 > | Example | Before 1.6.0 | Since 1.6.0 | Reason |
 > | --- | --- | --- | --- |
 > | `Gauge("foo_total")` | Rejected | Allowed | Not breaking because this previously failed at registration, so no working setup changes behavior, and safe because `_total` suffix expansion applies to counters, not gauges. |

docs/content/getting-started/registry.md

@@ -116,6 +116,14 @@ and full label-schema validation and duplicate detection still apply. A collecto
 non-null type but leaves `getLabelNames()` as `null` is still validated, with its labels treated as
 empty.
 
+This is also relevant for downstream adapter libraries that bridge to this registry. If an adapter
+implements `MultiCollector`, its registration-time metadata must match the metric families it will
+actually emit at scrape time. In practice, that means `getPrometheusNames()`, `getMetricType(...)`,
+`getLabelNames(...)`, and `getMetadata(...)` need to describe the same names, types, labels, and
+suffix behavior as the eventual `MetricSnapshot` output. Otherwise an adapter may pass or fail
+collision checks differently after upgrading to a newer client_java release, even if its scrape
+output logic did not change.
+
 ## Unregistering a Metric
 
 There is no automatic expiry of unused metrics (yet), once a metric is registered it will remain