make OTel endpoint optional and use defaults if note set

[cjs]

What

This change makes specifying an otel-endpoint optional. If not set, the OpenTelemetry exporters will be configured through:

• The OpenTelementry environment variables, if set
• The OpenTelemetry specification defaults

See https://github.com/open-telemetry/opentelemetry-go/tree/main/exporters/otlp/otlptrace#environment-variables and https://github.com/open-telemetry/opentelemetry-go/tree/main/exporters/jaeger#environment-variables for the defaults

Why

Setting the client with WithEndpoint forces the client to use the default URL Path. By making setting the endpoint via command line flag optional, we can unify the jaeger and otlp* provider command line flags, and still give the option of custom configurations via environment variables.

The alternative would be to add another flag for URLPath.

/cc authzed/spicedb#14
/cc @christroger

[jzelinskie]

This seems fine for now, but in the future when there could be more options, it might make more sense to do something like

var opts []jaeger.CollectorEndpointOption
if endpoint != "" {
    opts = append(opts, jaeger.WithEndpoint(endpoint))
}
exporter, err := jaeger.New(opts...)

This strategy can likely be applied to all of the other providers, too.

[cjs]

addressed in c4a5fd9

[christroger]

The additional changes I made, will allow us to set the Propagator to b3, ottrace and w3c. It is also possible to set multiple Propagators. To do that, the Propagators need to be separated by a comma. For example: --otel-trace-propagators "w3c,b3"
The default behaviour will use the w3c Propagators you already specified before.

[jzelinskie]

This should probably be MustGetString(cmd, prefixed("endpoint"))

[christroger]

True, updated it so the function uses the same Method to get the Flag value.

[cjs]

Sorry I missed this comment. This should not be MustGetString, since my intention was to make specifying the endpoint via the command line optional. The reasoning is WithEndpoint() does not allow configuration of the URL path, so if that is needed, configuration via environment variable is required.

[christroger]

In my Test, theMustGetString will just render an empty String as that is set as default.