TSDB: do not allocate exemplars buffer if exemplars are disabled (#8746)
TSDB: do not allocate exemplars buffer if exemplars are disabled (#8746)

Signed-off-by: Marco Pracucci <3829486b93ec44395f0b980424bae9b6fb07b7bc@pracucci.com>
alileza/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,prometheus/prometheus,keegancsmith/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,prometheus/prometheus,prometheus/prometheus,keegancsmith/prometheus,alileza/prometheus,GoogleCloudPlatform/prometheus,alileza/prometheus,keegancsmith/prometheus,GoogleCloudPlatform/prometheus,prometheus/prometheus,keegancsmith/prometheus,keegancsmith/prometheus