Stat sinks: Allow OpenTelemetry sink to export via HTTP

[joaopgrassi]

Title: Allow OpenTelemetry sink to export via HTTP

Description:
Similar to what was done for tracing, the stat sink for OpenTelemetry today only allow exporting via gRPC. This limits users to require either a back-end that handles OTLP/gRPC or deploy a OTel collector to do the conversion from gRPC to HTTP.

It would be nice if the stat sink could be configured to export via HTTP, allowing it to directly export to a back-end that supports OTLP/HTTP.

The sink is helpful, for example in troubleshooting to see if spans are being exported or not.

[optional Relevant Links:]

• HTTP should be the default one, according to the OTel spec

SDKs SHOULD support both grpc and http/protobuf transports and MUST support at least one of them. If they support only one, it SHOULD be http/protobuf. They also MAY support http/json.

open-telemetry/opentelemetry-java-instrumentation#10206

[phlax]

cc @AlexanderEllis @yanavlasov