Observability family — first release

Added

Document	What it carries
`reference/observability/cloud-logging.md`	five objects, the two default buckets, the routing sentence that enables a central logging project
`reference/observability/cloud-monitoring.md`	the eight concepts, the metrics scope, and an honest hedge on Prometheus
`reference/observability/cloud-trace.md`	traces and spans, OpenTelemetry as recommended, the two ingestion APIs
`reference/observability/errors.md`	six rows, each separating "the system is broken" from "the observability is broken"
`guides/observability/find-the-failing-call-in-cloud-logging.md`	six steps ending in a tested notification channel
`guides/parity/observability.md`	the one family whose estimate inputs are the same on all three clouds

The headline recommendation

Instrument with OpenTelemetry, on the source platform, before any migration. Cloud Trace recommends it over Google-specific APIs; AWS and Azure point at it too. It turns a per-service rewrite into an exporter change, and it pays for itself even if the migration never happens. It is the strongest sequencing recommendation in the whole hub.

What this release deliberately does not say

No log retention figures. The Cloud Logging overview states that entries "are stored for a specified

length of time and are then deleted" and puts the periods in a separate quotas document that was not read for this release. Quoting a remembered number here would be exactly the failure this hub exists to prevent.

No claim about Managed Service for Prometheus. The Cloud Monitoring page read references PromQL and does

not name that product, so reference/observability/cloud-monitoring.md §4 records the question rather than answering it — and says why the question is worth money in a migration.

No trace sampling behaviour. Named as a topic, not stated.

Known missing

Cloud Profiler and Error Reporting.
Managed Service for Prometheus in its own right.
Log Analytics on log buckets, and the Logging query language in detail.
Alerting policy syntax, SLO configuration, and synthetic monitors.
Audit log types and their configuration — referenced from the messaging family as an Eventarc prerequisite.