Renamed LB-related metadata keys

src/core/ext/lb_policy/grpclb/grpclb.c

src/core/ext/load_reporting/load_reporting.h

src/core/ext/load_reporting/load_reporting_filter.c

src/core/lib/transport/static_metadata.c

src/core/lib/transport/static_metadata.h

test/core/end2end/fuzzers/hpack.dictionary

test/core/end2end/tests/load_reporting_hook.c

test/cpp/grpclb/grpclb_test.cc

tools/codegen/core/gen_static_metadata.py