cloud_storage: Track hydrations in remote segment

In remote segment, when we start hydration of a segment or chunk, we
increment the active hydration counter. This is decremented when the
hydration finishes (successful or failed). This also includes the time
taken to acquire client etc so the time during which a hydration is
active is slightly longer than the actual download time span.

src/v/cloud_storage/remote_segment.cc

@@ -499,6 +499,7 @@ ss::future<> remote_segment::do_hydrate_segment() {
     auto reservation = co_await _cache.reserve_space(
       _size + storage::segment_index::estimate_size(_size), 1);
 
+    track_hydration t{_ts_probe};
     auto res = co_await _api.download_segment(
       _bucket,
       _path,
@@ -951,6 +952,8 @@ ss::future<> remote_segment::hydrate_chunk(segment_chunk_range range) {
       *this, std::move(range)};
 
     auto measurement = _ts_probe.chunk_hydration_latency();
+    track_hydration t{_ts_probe};
+
     auto res = co_await _api.download_segment(
       _bucket, _path, std::move(consumer), rtc, std::make_pair(start, end));
     if (res != download_result::success) {