Hoist constant setting out of loop - and shave some ns from call creation

src/core/ext/census/grpc_filter.c

src/core/ext/client_channel/client_channel.c

src/core/ext/load_reporting/load_reporting_filter.c

src/core/lib/channel/channel_stack.c

src/core/lib/channel/channel_stack.h

src/core/lib/channel/compress_filter.c

src/core/lib/channel/connected_channel.c

src/core/lib/channel/deadline_filter.c

src/core/lib/channel/http_client_filter.c

src/core/lib/channel/http_server_filter.c

src/core/lib/channel/message_size_filter.c

src/core/lib/security/transport/client_auth_filter.c

src/core/lib/security/transport/server_auth_filter.c

src/core/lib/surface/lame_client.c

src/core/lib/surface/server.c

test/cpp/microbenchmarks/bm_call_create.cc