HTTP.jl 2.0 reworked the server API: the `stream = true` mode of
`serve!` and the `setstatus` / `setheader` / `startwrite` streaming
helpers were removed in favor of returning an `HTTP.Response` from the
handler, and the running-server handle type changed from
`HTTP.Servers.Server` to `HTTP.Server`.

Rewrite the `/metrics` handler to buffer the body and return a
`Response` — a single code path that works on both HTTP 1.x and 2.x via
the still-supported `Router` / `register!` / `serve!` request-handler
API. Drop the now version-specific field type annotation on the stored
server handle. The pushgateway path (`HTTP.request`, `HTTP.Headers`) is
unchanged and already cross-version.

Widen the HTTP compat bound to "1, 2" and bump the package to 0.3.2.

Verified: the package's test suite (33 tests, including the end-to-end
`/metrics` HTTP round-trip) passes under both HTTP 1.x and HTTP 2.4.0.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

`Base.pkgversion(m)` can return `nothing` (its documented behavior when
a module wasn't loaded from a versioned package); `nothing >= v"2"`
would then throw at module load. Guard it and fall back to API feature
detection — `HTTP.Servers` exists throughout HTTP 1.x and was removed in
2.0 — which also covers Julia < 1.9 where `pkgversion` is unavailable.

Verified the primary and fallback branches agree on both HTTP 1.11 and
2.4, and that a full export round-trip (encode → POST → decode response
body) returns EXPORT_SUCCESS against a local collector on both versions.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>