The existing code tried to work around the complexities of optionally using
rvalue references' move capabilities if they exist. As seen in the previous
MapPair, there's a combinatorial explosion of prototypes to declare as the
parameter length increases. Because of this, addMetric ended up with a strange
API, and there was a wrapper to make a copy for insert.
Instead, we can apply the idiom of using universal/forwarding references and
std::forward to allow the compiler to instantiate the combinations that are
needed. There's a TagData struct with no copy constructor that validates that
move constructors can be properly called still.
I measured a 12-byte difference between before and after this change, so no
template bloat was introduced.
0ac868ad5d