Cleanup execution order workarounds

[swalkinshaw]

Until rmosolgo/graphql-ruby#3393, execute_multiplex wasn't guaranteed to be run first in all situations which mean we needed workarounds like

graphql-metrics/lib/graphql/metrics/tracer.rb

Lines 107 to 111 in fe0ef76

	def capture_parsing_time
	# GraphQL::Query#result fires `parse` before the `execute_multiplex` event, so sometimes
	# `pre_context.multiplex_start_time_monotonic` isn't set.
	parsing_offset_time = pre_context.multiplex_start_time_monotonic || GraphQL::Metrics.current_time_monotonic
	timed_result = GraphQL::Metrics.time(parsing_offset_time) { yield }

Once rmosolgo/graphql-ruby#3393 is released in a version, we could eventually bump our requirement and remove these workarounds. In the meantime, #48 clarifies documentation at least.