Prevent NPE when using otel metrics bridge asynchronously to agent start

[JonasKunz]

What does this PR do?

The elasticsearch team reported a NPE in the Otel metrics bridge, which is likely due to an asynchronous agent start.

java.lang.NullPointerException: Cannot invoke "io.opentelemetry.api.metrics.MeterProvider.get(String)" because "this.delegate" is null 
 at co.elastic.apm.agent.embeddedotel.proxy.ProxyMeterProvider.get(ProxyMeterProvider.java:36) ~[?:?]
 at co.elastic.apm.agent.opentelemetry.metrics.bridge.v1_14.BridgeMeterProvider.get(BridgeMeterProvider.java:41) ~[?:?]
 at io.opentelemetry.api.OpenTelemetry.getMeter(OpenTelemetry.java:102) ~[?:?]
 at org.elasticsearch.telemetry.apm.internal.APMMeterService.otelMeter(APMMeterService.java:90) ~[?:?]
 at org.elasticsearch.telemetry.apm.internal.APMMeterService.<init>(APMMeterService.java:33) ~[?:?]
 at org.elasticsearch.telemetry.apm.internal.APMTelemetryProvider.<init>(APMTelemetryProvider.java:22) ~[?:?]
 at org.elasticsearch.telemetry.apm.APM.getTelemetryProvider(APM.java:59) ~[?:?]
 at org.elasticsearch.node.NodeConstruction.lambda$createTelemetryProvider$1(NodeConstruction.java:475) ~[elasticsearch-8.15.2.jar:?]
 at java.util.Optional.map(Optional.java:260) ~[?:?]
 at org.elasticsearch.node.NodeConstruction.createTelemetryProvider(NodeConstruction.java:475) ~[elasticsearch-8.15.2.jar:?]
 at org.elasticsearch.node.NodeConstruction.prepareConstruction(NodeConstruction.java:258) ~[elasticsearch-8.15.2.jar:?]
 at org.elasticsearch.node.Node.<init>(Node.java:192) ~[elasticsearch-8.15.2.jar:?]
 at org.elasticsearch.bootstrap.Elasticsearch$2.<init>(Elasticsearch.java:242) ~[elasticsearch-8.15.2.jar:?]
 at org.elasticsearch.bootstrap.Elasticsearch.initPhase3(Elasticsearch.java:242) ~[elasticsearch-8.15.2.jar:?]
 at org.elasticsearch.bootstrap.Elasticsearch.main(Elasticsearch.java:76) ~[elasticsearch-8.15.2.jar:?]

This PR should prevent this exception and print warnings that a NoOp metrics implementation will be used in this case.

Checklist

• This is a bugfix
  • I have updated CHANGELOG.asciidoc
  • I have added tests that would fail without this fix