fix prometehus client exemplar panic

[liguozhong]

fix: grafana/loki#6667

const ExemplarMaxRunes = 64

	prometheus.Labels{"traceID": "008e1a5aa9165763983789310049f851"},

Runes = 39,just one traceId label ,no more label should be add.

[bboreham]

Thanks for this; sorry I didn't get to it sooner.
I merged #247 instead, which is a clean removal; I preferred that over commented-out code.

[liguozhong]

Thanks for this; sorry I didn't get to it sooner. I merged #247 instead, which is a clean removal; I preferred that over commented-out code.

No matter which PR is merged as long as it can solve the problem, this problem really bothered me last week