Add `oracle_cloud.realm` vendor attribute to Semantic Conventions

[geekdave]

Area(s)

area:cloud

What's missing?

Background

Back in #1658 we added Oracle Cloud Infrastructure (OCI) awareness to cloud.provider and cloud.platform but we didn't yet introduce any vendor-prefixed attributes.

We would like to introduce the first OCI vendor-specific attribute called realm.

What is a realm?

A realm is a logical collection of cloud regions that are isolated from each other and do not allow customer content to traverse realm boundaries to a region outside that realm. Each realm is accessed separately. OCI has multiple realms, including a commercial public cloud realm, multiple government cloud realms for the US and UK, and the Dedicated Regions from each customer are contained within their own separate realm.

-- From the Oracle Cloud Infrastructure Blog

Why is this needed?

Operators running across commercial and government environments need this context to route telemetry to realm-specific endpoints and enforce data residency/compliance.

More about realms:

1. https://docs.oracle.com/en-us/iaas/Content/General/Concepts/regions.htm
2. https://blogs.oracle.com/cloud-infrastructure/sovereign-cloud-realms-enhanced-isolation

Describe the solution you'd like

Add oracle_cloud.realm (type: string, stability: development) under a new vendor namespace (registry.oracle_cloud), documented with short examples (e.g., oc1, oc2) and a link to OCI realm docs. Keep it a free-form string to accommodate future realms.

Will be used by the oracle cloud resource detection processor to populate the realm attribute. See open-telemetry/opentelemetry-collector-contrib#44408

cc @dashpole @tonychoe

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