Support `OTEL_EXPORTER_OTLP_PROTOCOL` et al

[pellared]

Add support for following env vars:

• OTEL_EXPORTER_OTLP_PROTOCOL
• OTEL_EXPORTER_OTLP_TRACES_PROTOCOL
• OTEL_EXPORTER_OTLP_METRICS_PROTOCOL
• OTEL_EXPORTER_OTLP_LOGS_PROTOCOL

These need to default to grpc by default for backwards-compatibility.

[pellared]

This code

splunk-otel-go/distro/exporter.go

Lines 77 to 93 in de82774

	var opts []otlptracegrpc.Option
	if c.AccessToken != "" {
	opts = append(opts, otlptracegrpc.WithHeaders(map[string]string{
	"X-Sf-Token": c.AccessToken,
	}))
	}
	if c.TLSConfig != nil {
	tlsCreds := credentials.NewTLS(c.TLSConfig)
	opts = append(opts, otlptracegrpc.WithTLSCredentials(tlsCreds))
	} else if noneEnvVarSet(otelExporterOTLPEndpointKey, otelExporterOTLPTracesEndpointKey, splunkRealmKey) {
	// Assume that the default endpoint (local collector) is non-TLS.
	opts = append(opts, otlptracegrpc.WithTLSCredentials(insecure.NewCredentials()))
	}
	return otlptracegrpc.New(ctx, opts...)

should setup an otlptracehttp exporter if OTEL_EXPORTER_OTLP_PROTOCOL=http/protobuf or OTEL_EXPORTER_OTLP_TRACES_PROTOCOL=http/protobuf.

Similarly for metrics and logs.

[ysolomchenko]

I’ll work on this