Add GC metric last_incremental_sweep (JuliaLang#50190)

* Add GC metric `last_incremental_sweep`

* Update gc.c

* Update gc.c

Author: li1

base/timing.jl

@@ -25,6 +25,7 @@ struct GC_Num
     total_sweep_time  ::Int64
     total_mark_time   ::Int64
     last_full_sweep ::Int64
+    last_incremental_sweep ::Int64
 end
 
 gc_num() = ccall(:jl_gc_num, GC_Num, ())

src/gc.c

@@ -3301,6 +3301,9 @@ static int _jl_gc_collect(jl_ptls_t ptls, jl_gc_collection_t collection)
     if (sweep_full) {
         gc_num.last_full_sweep = gc_end_time;
     }
+    else {
+        gc_num.last_incremental_sweep = gc_end_time;
+    }
 
     int thrashing = 0; // maybe we should report this to the user or error out?
     size_t heap_size = jl_atomic_load_relaxed(&gc_heap_stats.heap_size);

src/gc.h

@@ -83,6 +83,7 @@ typedef struct {
     uint64_t    total_sweep_time;
     uint64_t    total_mark_time;
     uint64_t    last_full_sweep;
+    uint64_t    last_incremental_sweep;
 } jl_gc_num_t;
 
 // Array chunks (work items representing suffixes of