fix otel custom provider sample

[ryota0624]

expose metrics require calling otel.SetMeterProvider(meterProvider)

before (none calling otel.SetMeterProvider(meterProvider))

1:01PM INF actor system started lib=Proto.Actor system=J8MjnjRbMnrMqwaRdKNLBg id=J8MjnjRbMnrMqwaRdKNLBg
{"Resource":[{"Key":"service.name","Value":{"Type":"STRING","Value":"test-service"}},{"Key":"service.version","Value":{"Type":"STRING","Value":"0.1.0"}},{"Key":"telemetry.sdk.language","Value":{"Type":"STRING","Value":"go"}},{"Key":"telemetry.sdk.name","Value":{"Type":"STRING","Value":"opentelemetry"}},{"Key":"telemetry.sdk.version","Value":{"Type":"STRING","Value":"1.22.0"}}],"ScopeMetrics":null}

after

1:01PM INF actor system started lib=Proto.Actor system=4zqzpEKUr5MvsLe7aHhdLg id=4zqzpEKUr5MvsLe7aHhdLg
Hello Stdout Exporter
{"Resource":[{"Key":"service.name","Value":{"Type":"STRING","Value":"test-service"}},{"Key":"service.version","Value":{"Type":"STRING","Value":"0.1.0"}},{"Key":"telemetry.sdk.language","Value":{"Type":"STRING","Value":"go"}},{"Key":"telemetry.sdk.name","Value":{"Type":"STRING","Value":"opentelemetry"}},{"Key":"telemetry.sdk.version","Value":{"Type":"STRING","Value":"1.22.0"}}],"ScopeMetrics":[{"Scope":{"Name":"protoactor","Version":"","SchemaURL":""},"Metrics":[{"Name":"protoactor_actor_message_receive_duration_seconds","Description":"Actor's messages received duration in seconds","Unit":"","Data":{"DataPoints":[{"Attributes":[{"Key":"actortype","Value":{"Type":"STRING","Value":"main.helloActor"}},{"Key":"address","Value":{"Type":"STRING","Value":"nonhost"}},{"Key":"messagetype","Value":{"Type":"STRING","Value":"*actor.Started"}}],"StartTime":"2025-01-03T13:01:49.9814+09:00","Time":"2025-01-03T13:01:52.979789+09:00","Count":1,"Bounds":[0,5,10,25,50,75,100,250,500,750,1000,2500,5000,7500,10000],"BucketCounts":[0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0],"Min":{},"Max":{},"Sum":0.000004958},{"Attributes":[{"Key":"actortype","Value":{"Type":"STRING","Value":"main.helloActor"}},{"Key":"address","Value":{"Type":"STRING","Value":"nonhost"}},{"Key":"messagetype","Value":{"Type":"STRING","Value":"*main.hello"}}],"StartTime":"2025-01-03T13:01:49.9814+09:00","Time":"2025-01-03T13:01:52.979789+09:00","Count":1,"Bounds":[0,5,10,25,50,75,100,250,500,750,1000,2500,5000,7500,10000],"BucketCounts":[0,1,0,0,0,0,0,0,0,0,0,0,0,0,0,0],"Min":{},"Max":{},"Sum":0.000003291}],"Temporality":"CumulativeTemporality"}},{"Name":"protoactor_actor_spawn_count","Description":"Number of actors spawn","Unit":"1","Data":{"DataPoints":[{"Attributes":[{"Key":"actortype","Value":{"Type":"STRING","Value":"main.helloActor"}},{"Key":"address","Value":{"Type":"STRING","Value":"nonhost"}}],"StartTime":"2025-01-03T13:01:49.981409+09:00","Time":"2025-01-03T13:01:52.979805+09:00","Value":1}],"Temporality":"CumulativeTemporality","IsMonotonic":true}}]}]}

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