fix: panic when multiple hcloud-go Client with Instrumentation are registered

[apricote]

If one creates multiple Clients WithInstrumentation on the same registry, this will cause a panic because the Collectors (by name) are already defined from a previous Client.

panic: duplicate metrics collector registration attempted

goroutine 16 [running]:
github.com/prometheus/client_golang/prometheus.(*Registry).MustRegister(...)
	/home/x/go/pkg/mod/github.com/prometheus/client_golang@v1.16.0/prometheus/registry.go:405
github.com/hetznercloud/hcloud-go/v2/hcloud/internal/instrumentation.(*Instrumenter).InstrumentedRoundTripper(0xc0001bbcf0)
	/home/x/go/pkg/mod/github.com/hetznercloud/hcloud-go/v2@v2.0.0/hcloud/internal/instrumentation/metrics.go:48 +0x785
github.com/hetznercloud/hcloud-go/v2/hcloud.NewClient({0xc0001bbd98, 0x3, 0x10?})
	/home/x/go/pkg/mod/github.com/hetznercloud/hcloud-go/v2@v2.0.0/hcloud/client.go:189 +0x273