[inputs.opentelemetry] should support HTTP (and not only gRPC)

### Use Case

There are products which can only send OpenTelemetry metrics via HTTP (and not gRPC).

### Expected behavior

`[inputs.opentelemetry]` should support HTTP(S) input address.

### Actual behavior

`[inputs.opentelemetry]` currently only support gRPC input.

### Additional info

OTLP specification does specify gRPC and HTTP: https://opentelemetry.io/docs/specs/otlp/#otlphttp