add more OTEL configuration options

[hgiasac]

• Improve OTEL configuration options. The SDK no longer detects HTTP protocol via http(s) scheme in the URL. By default, the OTEL exporter uses GRPC protocol. You can change to HTTP protocol by using port 4318 or set OTEL_EXPORTER_OTLP_PROTOCOL=http/protobuf
• Now you can choose only one metrics exporter by OTEL_METRICS_EXPORTER=<none|otlp|prometheus> variable. By default metrics exporter is disabled (none).
• If OTEL_METRICS_EXPORTER != prometheus, the /metrics route returns HTTP 404. You can change Prometheus service to another port with OTEL_EXPORTER_PROMETHEUS_PORT variable.
• Bad request HTTP status is changed to HTTP 422. Engine v3 only exposes errors with this status (source).

[github-actions]

Package	Line Rate	Health
github.com/hasura/ndc-sdk-go/cmd/ndc-go-sdk	74%	✔
github.com/hasura/ndc-sdk-go/connector	55%	➖
github.com/hasura/ndc-sdk-go/example/reference	63%	➖
github.com/hasura/ndc-sdk-go/internal	46%	➖
github.com/hasura/ndc-sdk-go/schema	39%	❌
github.com/hasura/ndc-sdk-go/utils	43%	➖
Summary	51% (3591 / 7052)	➖

Minimum allowed line rate is 40%