Block `Collect`/`Export` after `Shutdown`

Spec requirement

MetricReader#Shutdown MUST be called once; after it, subsequent Collect calls MUST NOT be allowed and SHOULD fail (sdk.md:1524-1539). The same applies to MetricExporter#Shutdown/Export (sdk.md:1757-1769).

Current behavior

export/metric_reader.rb:25-27 (permalink) records no state, so collect remains callable (and functional) after shutdown. Separately, the OTLP MetricsExporter#shutdown (metrics_exporter.rb:387-390 (permalink)) sets @shutdown = true, but that flag is never read elsewhere in the file. export also remains functional after shutdown.

Suggested fix

Track a @stopped flag on the base MetricReader and check it at the top of collect, returning a failure/no-op result once set (mirroring ConsoleMetricPullExporter's existing @stopped pattern). Add the equivalent guard clause to the OTLP exporter's export/send_bytes.

Related rows in SPEC_COMPLIANCE_METRICS.md

SDK-56, OTLP-11

Start with export/metric_reader.rb:25-27 and the existing @stopped pattern in ConsoleMetricPullExporter. Then inspect metrics_exporter.rb:387-390 and its export/send_bytes flow. Add shutdown guards so Collect and Export no longer operate after shutdown, and verify the behavior against SDK-56 and OTLP-11 in SPEC_COMPLIANCE_METRICS.md.