fix: always use sum aggregator for sum & count

[raghuram-periaswamy]

sum & count metrics of summary & histogram should always be summed irrespective of the custom aggregator that's mentioned as part of the metric definition.

Previously, aggregating the following two sample sets

set-1

http_duration_seconds{quantile="0.95"} 4
http_duration_seconds_sum 10
http_duration_seconds_count 5

set-2

http_duration_seconds{quantile="0.95"} 6
http_duration_seconds_sum 20
http_duration_seconds_count 5

using average yields the following where every metric is averaged.

http_duration_seconds{quantile="0.95"} 5
http_duration_seconds_sum 15
http_duration_seconds_count 5

The fix will always perform sum for the sum & count yielding,

http_duration_seconds{quantile="0.95"} 5
http_duration_seconds_sum 30
http_duration_seconds_count 10

[raghuram-periaswamy]

@SimenB @zbjornson Requesting for the review.