Add OTEL_EXPORTER_OTLP_LOGS_INSECURE and OTEL_EXPORTER_OTLP_INSECURE exporter env options

[amanakin]

In this commit I add OTEL_EXPORTER_OTLP_LOGS_INSECURE and OTEL_EXPORTER_OTLP_INSECURE env options to otlploggrpc.Exporter.

Now insecure option is fetched from env endpoint value (OTEL_EXPORTER_OTLP_LOGS_ENDPOINT/OTEL_EXPORTER_OTLP_ENDPOINT). According to spec:

Insecure: Whether to enable client transport security for the exporter’s gRPC connection. This option only applies to OTLP/gRPC when an endpoint is provided without the http or https scheme - OTLP/HTTP always uses the scheme provided for the endpoint.

So with current behavior we have several problems:

• If default endpoint is used, we can't use insecure connection (with setting OTEL_EXPORTER_OTLP_INSECURE).
• If endpoint provided with option without scheme (e.g. WithEndpoint) we can't use insecure connection with env settings.
• If endpoint provided with env variable without scheme (e.g. //env.endpoint:8080/) we can't use insecure connection.

This commit fixes this.

The same problem with otlploghttp.Exporter, and probably it should be fixed there too. I'm open to suggestions on how to fix the current behavior in a more elegant way.