Add inflight memory counters for search

[fulmicoton]

Ideally we would like to have a decent picture of the memory usage between

• ephemeral cache
• fast field cache
• hotcache
• buffers if relevant ?

[trinity-1686a]

this sounds a lot like quickwit_cache_in_cache_num_bytes

# HELP quickwit_cache_in_cache_num_bytes Number of bytes in cache by component
# TYPE quickwit_cache_in_cache_num_bytes gauge
quickwit_cache_in_cache_num_bytes{component_name="fastfields"} 0
quickwit_cache_in_cache_num_bytes{component_name="fd"} 0
quickwit_cache_in_cache_num_bytes{component_name="partial_request"} 0
quickwit_cache_in_cache_num_bytes{component_name="searcher_split"} 0
quickwit_cache_in_cache_num_bytes{component_name="shortlived"} 0
quickwit_cache_in_cache_num_bytes{component_name="splitfooter"} 0

is there more to add beside that?