Backport datadog `span_type` change to 2.0.x #1979

Job	Run time
lint	32s
javascript_test	54s
postgres_test	1m 17s
mongodb_test (gemfiles/mongoid_6.gemfile)	1m 3s
mongodb_test (gemfiles/mongoid_7.gemfile)	1m 1s
system_tests	48s
test (Gemfile, 3.1)	42s
test (gemfiles/rails_4.2.gemfile, 2.4, 1)	27s
test (gemfiles/rails_5.2.gemfile, 2.7)	57s
test (gemfiles/rails_7.0.gemfile, 3.1)	51s
test (gemfiles/rails_master.gemfile, 3.1)	1m 10s
test (gemfiles/rails_7.0.gemfile, 3.2)	50s
test (gemfiles/rails_master.gemfile, 3.2)	1m 12s
	11m 44s

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