[BUILD] Build break when using OTLP HTTP Metrics exporter

[marcalff]

In an application, include file
opentelemetry/exporters/otlp/otlp_http_metric_exporter.h,
to see class OtlpHttpMetricExporter

File opentelemetry/exporters/otlp/otlp_metric_utils.h is included in the build.

The build fails:

fatal error: 'opentelemetry/proto/collector/metrics/v1/metrics_service.pb.h' file not found

because protobuf generated files are not installed with opentelemetry headers.

There are two issues here.

opentelemetry-cpp should expose Factory::Create() methods to build instances of OtlpHttpMetricExporter, per issue #1485, so that users don't need to see OtlpHttpMetricExporter directly.

Because the factory is not available, the work around is to instantiate OtlpHttpMetricExporter instead, which leads to:

The class definition in otlp_http_metric_exporter.h should not need to see protobuf generated code, this is an implementation dependency.