Please describe the feature request.
Provide configuration properties to provide custom headers e.g. API-Key which are sent whenever metrics are posted to the OTLP endpoint.

Having something like this

management:
  otlp:
    metrics:
      export:
        step: 1s
        url: https://otlp.eu01.nr-data.net/v1/metrics
        headers:
          API-Key: otlp-endpoint-api-key

so that these headers are sent with each request may be this way

var builder = this.httpSender.post(this.config.url());
for (final var header : this.config.headers()) {
  builder = builder.withHeader(header.key(), header.value());
}
builder.withContent("application/x-protobuf", request.toByteArray()) .send();

see

Another solution would be to use the Open Telemetry SDK and allow to provide custom MetricExporter beans. This works very well already for tracing. Here I can provide custom SpanExporter beans.

Rationale
Having direct OTLP support is great but in order to connect with some OTLP endpoints, e.g. NewRelics OTLP endpoints a header containing an API key has to be sent with each request.