chore: Add call to ENV.fetch (rubocop offense) (#2188)

Author: kaylareopelle

exporter/otlp-http/lib/opentelemetry/exporter/otlp/http/trace_exporter.rb

@@ -260,7 +260,7 @@ def prepare_headers(config_headers)
           end
 
           def prepare_endpoint(endpoint)
-            endpoint ||= ENV['OTEL_EXPORTER_OTLP_TRACES_ENDPOINT']
+            endpoint ||= ENV.fetch('OTEL_EXPORTER_OTLP_TRACES_ENDPOINT', nil)
             if endpoint.nil?
               endpoint = ENV['OTEL_EXPORTER_OTLP_ENDPOINT'] || 'http://localhost:4318'
               endpoint += '/' unless endpoint.end_with?('/')