Allow to fallback on global OTEL options

[olegbespalov]

What?

As pointed out in grafana/xk6-output-opentelemetry#24, how we currently define our default configuration makes it harder to use the output and confuses users familiar with OTEL SDK configuration.

The current approach was likely adopted to provide k6-specific configurability. However, it creates a divergence from standard OTEL practices, potentially confusing users familiar with OTEL SDK configuration.
Moving forward, we should consider:

• Gradually aligning our custom variables with standard OTEL naming conventions.
• Implementing a fallback mechanism where standard OTEL variables are checked if custom ones are not set.
• Providing clear documentation on which standard OTEL variables are supported and which are superseded by custom configurations.

So, we must adjust our configuration and consider OTEL environment variables if they are defined. Still K6_OTEL_ will have priority.

Why?

Align with OTEL.

[TimotejKovacka]

Upon further review, I've reconsidered my initial assessment which was overly focused on OpenTelemetry compatibility. Since the extension currently follows K6's standard extension patterns, it likely doesn't require modifications to maintain compatibility with OpenTelemetry environment variables. This approach aligns with how other K6 extensions handle similar scenarios.

What could be improved instead is the UX that could come with warning's to the User when a miss match happens like pointed out in #4070:

K6_OTEL_EXPORTER_TYPE=http \
K6_OTEL_GRPC_EXPORTER_ENDPOINT=prometheus:9090 \
K6_OTEL_HTTP_EXPORTER_URL_PATH=/api/v1/otlp/v1/metrics \
K6_OTEL_METRIC_PREFIX=agilob \
./k6 run --out experimental-opentelemetry main.js

The system should warn users when they've provided a GRPC exporter endpoint (K6_OTEL_GRPC_EXPORTER_ENDPOINT) while the exporter type is set to HTTP (K6_OTEL_EXPORTER_TYPE=http). This would help users identify configuration conflicts earlier and prevent confusion about which settings are actually being applied. This could be communicated in a fashion like so:

⚠️ Warning: Configuration mismatch detected
You've set HTTP exporter type (K6_OTEL_EXPORTER_TYPE=http) but provided a GRPC endpoint (K6_OTEL_GRPC_EXPORTER_ENDPOINT=prometheus:9090).

- To use GRPC, set K6_OTEL_EXPORTER_TYPE=grpc