shrink string cache reset threshold a bit

A slow ramp up to 4 MiB could _look_ like a memory leak. However, a
slow ramp to 2 MiB is probably going to look like it's within normal
operating ranges.

profiling/src/profiling/stack_walking.rs

@@ -172,8 +172,10 @@ mod detail {
         CACHED_STRINGS.with(|cell| {
             let set: &StringSet = &cell.borrow();
             let arena_used_bytes = set.arena_used_bytes();
-            // todo: pin down threshold
-            let threshold = 4 * 1024 * 1024;
+            // A slow ramp up to 2 MiB is probably _not_ going to look like
+            // a memory leak, whereas a higher threshold could make a user
+            // suspect a leak.
+            let threshold = 2 * 1024 * 1024;
             if arena_used_bytes > threshold {
                 debug!("string cache arena is using {arena_used_bytes} bytes which exceeds the {threshold} byte threshold, resetting");
                 // Note that this cannot be done _during_ a request. The

profiling/src/string_set.rs

@@ -35,7 +35,7 @@ impl StringSet {
         // Keep this in the megabyte range. It's virtual, so we do not need
         // to worry much about unused amounts, but asking for wildly too much
         // up front, like in gigabyte+ range, is not good either.
-        const SIZE_HINT: usize = 4 * 1024 * 1024;
+        const SIZE_HINT: usize = 2 * 1024 * 1024;
         let arena = ChainAllocator::new_in(SIZE_HINT, VirtualAllocator {});
 
         let mut strings = HashSet::with_hasher(Hasher::default());