Inline (by copy-paste) the fast path of `decodeVarint` in various
places where it gets called a lot.
Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
The individual strings for label names and values are held in a table,
and each Labels value is a run of varint-encoded indexes into that table.
When creating new labels, a sync.Mutex is locked around reads and writes.
When reading labels, there is no locking because the table of strings
used by those labels is immutable.
Signed-off-by: Bryan Boreham <bjboreham@gmail.com>
