Merge pull request #11727 from ncteisen/the-tracers-become-self-aware

doc/environment_variables.md

src/core/ext/filters/client_channel/client_channel.c

src/core/ext/filters/client_channel/client_channel_plugin.c

src/core/ext/filters/client_channel/lb_policy.c

...core/ext/filters/client_channel/lb_policy/grpclb/grpclb.c

.../filters/client_channel/lb_policy/pick_first/pick_first.c

...ilters/client_channel/lb_policy/round_robin/round_robin.c

src/core/ext/filters/client_channel/resolver.c

src/core/ext/filters/http/http_filters_plugin.c

src/core/ext/transport/chttp2/transport/chttp2_plugin.c

src/core/ext/transport/chttp2/transport/chttp2_transport.c

src/core/lib/channel/channel_stack.c

src/core/lib/channel/channel_stack_builder.c

src/core/lib/debug/trace.c

src/core/lib/debug/trace.h

src/core/lib/http/parser.c

src/core/lib/iomgr/closure.c

src/core/lib/iomgr/combiner.c

src/core/lib/iomgr/error.c

src/core/lib/iomgr/ev_posix.c