buildSamplerFromEnv() in sdk-trace-base package should use parentbased_always_on if the given OTEL_TRACES_SAMPLER is unknown

[trentm]

Per https://opentelemetry.io/docs/specs/otel/configuration/sdk-environment-variables/#general-sdk-configuration the default for OTEL_TRACES_SAMPLER is "parentbased_always_on".

Current process.env handling for OTEL_TRACES_SAMPLER does default to that value if the envvar is not specified, or empty. However, if a value is given but is unknown, the code is falling back to always_on instead of parentbased_always_on:

opentelemetry-js/packages/opentelemetry-sdk-trace-base/src/config.ts

Lines 98 to 102 in c27fbc6

	default:
	diag.error(
	`OTEL_TRACES_SAMPLER value "${environment.OTEL_TRACES_SAMPLER} invalid, defaulting to ${FALLBACK_OTEL_TRACES_SAMPLER}".`
	);
	return new AlwaysOnSampler();