Use WithExplicitBucketBoundaries in the prometheus example

[dashpole]

Users no longer need to use views to match the behavior of prometheus.NewHistogramVec.

Tested by manually verifying the prometheus output:

# HELP baz a histogram with custom buckets and rename
# TYPE baz histogram
baz_bucket{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version="",le="64"} 1
baz_bucket{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version="",le="128"} 1
baz_bucket{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version="",le="256"} 2
baz_bucket{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version="",le="512"} 2
baz_bucket{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version="",le="1024"} 4
baz_bucket{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version="",le="2048"} 4
baz_bucket{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version="",le="4096"} 4
baz_bucket{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version="",le="+Inf"} 4
baz_sum{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version=""} 1731
baz_count{A="B",C="D",otel_scope_name="github.com/open-telemetry/opentelemetry-go/example/prometheus",otel_scope_version=""} 4

This is a minor cleanup in an example, so skipping changelog