Fix View `attribute_keys` to behave as an allowlist (with exclude list)

Spec requirement

Stream configuration's attribute_keys MUST behave as an allowlist — "keys that MUST be kept, all others MUST be ignored" — and SHOULD also support an exclude list (sdk.md:359-461, Stream configuration).

Current behavior

state/metric_stream.rb:91 (permalink) does attributes.merge!(view.attribute_keys). This adds/overwrites the given key-value pairs onto the measurement's attributes rather than filtering the measurement down to only the listed keys. Confirmed by test fixtures passing extra attributes (attribute_keys: { 'environment' => 'test' } in test/opentelemetry/sdk/metrics/state/metric_stream_test.rb:97). In addition, there is no exclude list support.

Suggested fix

Change attribute_keys handling to filter the measurement's attribute hash down to only the configured keys (attributes.slice(*view.attribute_keys) or equivalent), and add a separate exclude list option that removes named keys instead. This is potentially a breaking change fix. Anyone currently relying on attribute_keys to add literal values will need a different mechanism once this ships.

Related rows in SPEC_COMPLIANCE_METRICS.md

SDK-13

Start at metrics_sdk/lib/opentelemetry/sdk/metrics/state/metric_stream.rb:91 and inspect the related fixtures in test/opentelemetry/sdk/metrics/state/metric_stream_test.rb:97. Verify the configured attribute_keys retain only matching measurement attributes, add the specified exclude-list behavior, and update tests so extra attributes and literal-value assumptions no longer pass.