feat: Add authenticated environment attributes to relay.auth span and metrics

[keelerm84]

Summary

Emit the authenticated environment as telemetry attributes so traces and metrics can be correlated by environment.

• relay.auth span: on successful auth, both auth middlewares now set relay.auth.environment.name (always, the human-readable display name) and relay.auth.environment.id (only when an EnvironmentID credential is configured). The relay.auth.result=success attribute remains unconditional.
• Metrics: the per-environment attribute set keeps the existing environment.name and now also emits environment.id when available.

The environment ID may be absent for SDK-key-only environments in a manual configuration, so it is only added when present.

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Note

Low Risk
Additive telemetry and an internal metrics API signature change; no auth or request-handling behavior changes beyond extra span attributes.

Overview
Adds environment identity to OpenTelemetry so traces and per-environment metrics can be correlated beyond display name alone.

On successful auth, both SelectEnvironmentByAuthorizationKey and SelectEnvironmentByClientSideAuth set relay.auth.environment.name (display name) and, when configured, relay.auth.environment.id on the relay.auth span via a shared authEnvSpanAttributes helper. environment.id is omitted for SDK-key-only manual configs where no environment ID exists.

Metrics extend Manager.AddEnvironment with an envID argument; the environment attribute set still includes environment.name and now adds environment.id when non-empty. Environment startup passes envConfig.EnvID from env_context_impl.

Tests cover presence/absence of environment.id on metric attributes; existing AddEnvironment call sites pass an empty ID where appropriate.

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