Cache label name and last value when adding series (#8192)
Cache label name and last value when adding series (#8192)

Signed-off-by: Chris Marchbanks <0ad0b538c51a24737b83a297dff91636ee98d89e@gmail.com>
GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,keegancsmith/prometheus,prometheus/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,alileza/prometheus,keegancsmith/prometheus,alileza/prometheus,prometheus/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus