Since it's possible to have multiple uses of httpsnoop in a project, coming from different imported modules (e.g. when using server-timing), it's also possible to end up wrapping a ResponseWriter multiple times. This is not ideal because it has a sensible performance impact:

cpu: Intel(R) Core(TM) i7-8565U CPU @ 1.80GHz
BenchmarkBaseline
BenchmarkBaseline-8                     745539247                1.477 ns/op           0 B/op          0 allocs/op
BenchmarkCaptureMetrics
BenchmarkCaptureMetrics-8                3790958               296.2 ns/op           225 B/op          7 allocs/op
BenchmarkCaptureMetricsTwice
BenchmarkCaptureMetricsTwice-8           1912039               581.3 ns/op           450 B/op         14 allocs/op

(these benchmarks were made after switching httptest.NewServer for direct h.ServeHTTP calls, in order to avoid the unrelated overhead of the server; see #20 )

Ideally we'd return the same Metrics instance when re-wrapping an already wrapped ResponseWriter.

In case we explicitly want to measure at two different places in the middleware-chain, we'd still be able to Unwrap before re-wrapping.

WDYT?