Fix appending custom values to specs

[NelsonVides]

Fixes a bug introduced at #181 when we introduced maps as specs, as histograms and summaries would append values as if they were still lists.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Files with missing lines	Coverage Δ
src/metrics/prometheus_histogram.erl	95.72% <100.00%> (ø)
src/metrics/prometheus_quantile_summary.erl	92.36% <100.00%> (+0.05%)	⬆️
test/eunit/metric/prometheus_histogram_tests.erl	98.28% <100.00%> (+0.05%)	⬆️
...eunit/metric/prometheus_quantile_summary_tests.erl	98.13% <100.00%> (+0.08%)	⬆️

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