Load Reporting back to using metadata

grpc.def

include/grpc/grpc.h

include/grpc/load_reporting.h

src/core/ext/filters/load_reporting/load_reporting.c

src/core/ext/filters/load_reporting/load_reporting_filter.c

src/core/lib/channel/context.h

src/core/lib/transport/static_metadata.c

src/core/lib/transport/static_metadata.h

src/cpp/server/server_context.cc

src/ruby/ext/grpc/rb_grpc_imports.generated.c

src/ruby/ext/grpc/rb_grpc_imports.generated.h

test/core/end2end/fuzzers/hpack.dictionary

test/core/end2end/tests/load_reporting_hook.c

tools/codegen/core/gen_static_metadata.py