Make `add_view` apply retroactively to existing instruments

Spec requirement

If configuration is updated (e.g. adding a View), the update MUST also apply to all already-returned Meters/instruments (sdk.md:143-159, Configuration).

Current behavior

add_metric_reader (meter_provider.rb:114-122 (permalink)) retroactively pushes the new reader onto every existing Meter. add_view (meter_provider.rb:186-189 (permalink)) does not. MetricStream#find_registered_view (state/metric_stream.rb:123-127 (permalink)) only runs once, at stream construction, so a View added after an instrument's first measurement won't affect that instrument's already-created stream.

Suggested fix

Have add_view re-resolve the matching View for every existing MetricStream, or re-run find_registered_view lazily on each collect/update rather than caching it once at construction.

Related rows in SPEC_COMPLIANCE_METRICS.md

SDK-6

Start with metrics_sdk/lib/opentelemetry/sdk/metrics/meter_provider.rb, especially add_view, then trace existing streams through metrics_sdk/lib/opentelemetry/sdk/metrics/state/metric_stream.rb and find_registered_view. Verify the change against an instrument and stream created before a View is added; done means the later View affects that existing instrument as required by SDK-6.