[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