README: document the OpenTelemetry OTLP push bridge (#1987)

Adds a short section pointing users at
`go.opentelemetry.io/contrib/bridges/prometheus`, which wraps a
`prometheus.Registry` so client_golang-instrumented applications can
push metrics via OTLP without re-instrumenting against a different
metric API. Keeps the upstream recommendation that pull-based
`promhttp` is still the default; the bridge is positioned as an
additional option for batch jobs and push-only environments.

Closes #1495.

Signed-off-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Matt Van Horn <455140+mvanhorn@users.noreply.github.com>
Co-authored-by: Kemal Akkoyun <kakkoyun@users.noreply.github.com>