otlp: treat OTEL_EXPORTER_OTLP_ENDPOINT='' as OpenTelemetry being disabled
Created by: bobheadxi
We need a mechanism to disable OpenTelemetry by default even if it is enabled by default (#41242), most notably for sourcegraph/server
(https://github.com/sourcegraph/sourcegraph/pull/41244)
Test plan
OTEL_EXPORTER_OTLP_ENDPOINT='' sg start
[ worker] WARN tracer tracer/tracer.go:163 failed to initialize tracer {"tracerType": "opentelemetry", "debug": false, "error": "please configure an exporter endpoint with OTEL_EXPORTER_OTLP_ENDPOINT"}