Allow disabling traces, add trace variables for pluck and timeout events

BUILD

build.yaml

gRPC.podspec

grpc.gemspec

package.xml

src/core/lib/debug/trace.c

src/core/lib/surface/call.h

src/core/lib/surface/completion_queue.c

src/core/lib/surface/completion_queue.h

src/core/lib/surface/init.c

src/core/lib/surface/surface_trace.h

tools/doxygen/Doxyfile.core.internal

tools/run_tests/sources_and_headers.json

vsprojects/vcxproj/grpc/grpc.vcxproj

vsprojects/vcxproj/grpc/grpc.vcxproj.filters

vsprojects/vcxproj/grpc_unsecure/grpc_unsecure.vcxproj

...jects/vcxproj/grpc_unsecure/grpc_unsecure.vcxproj.filters