otlpmetric: Support OTEL_EXPORTER_OTLP_PROTOCOL and OTEL_EXPORTER_OTLP_METRICS_PROTOCOL environmental variables

[pellared]

Reference open-telemetry/opentelemetry-specification#3721 (comment)

[pellared]

I think that it should really follow the database/sql driver pattern and require the users to call e.g. import _ "http://go.opentelemetry.io/otel/exporters/otlp/otlpmetric/otlpmetricgrpc" if one want to have support grpc for OTEL_EXPORTER_OTLP_PROTOCOL and OTEL_EXPORTER_OTLP_METRICS_PROTOCOL env vars.

I propose to add

• the protocol client factory (driver) that is invoked when given protocol is told to be created

type Driver interface {
	New() Client
}

• a registry function that is used to set a driver for OTEL_EXPORTER_OTLP_PROTOCOL and OTEL_EXPORTER_OTLP_METRICS_PROTOCOL env var value; this function would be used by init functions in otlptracehttp and otlptracegrpc

func RegisterProtocol(key string, driver Driver)

• the New and NewUnstarted functions use the protocol drivers if a nil client is passed

The same pattern should be used for OTEL_METRICS_EXPORTER support. Users would have to call e.g. import _ "go.opentelemetry.io/otel/exporters/otlp/otlpmetric" if one want to have support otlp for OTEL_METRICS_EXPORTER env var.

The exported API should for traces and metrics should be similar.

[pellared]

In parallel, I plan to work on #4642, #4640, #2310 to have an API which would be consistent for all modules.

Probably I will start with a PoC draft PR to showcase the proposed API.

[pellared]

On hold. See open-telemetry/opentelemetry-specification#3721 (comment)

[pellared]

Closing per open-telemetry/opentelemetry-specification#3730