Backport datadog `span_type` change to 1.13.x #1945

Job	Run time
legacy_test	4m 40s
mongodb_test (gemfiles/mongoid_6.gemfile)	1m 38s
lint	1m 4s
mongodb_test (gemfiles/mongoid_7.gemfile)	1m 29s
javascript_test	1m 18s
postgres_test	2m 10s
system_tests	54s
test (Gemfile, 3, 1)	1m 7s
test (gemfiles/rails_4.2.gemfile, 2.4, 1)	1m 36s
test (gemfiles/rails_5.2.gemfile, 2.7)	2m 41s
test (gemfiles/rails_7.0.gemfile, 3)	1m 21s
test (gemfiles/rails_master.gemfile, 3)	1m 2s
	21m 0s

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