Reduce metrics collection overhead #1024

What is the problem the feature request solves?

Running TPC-H locally, I see >3% of on-CPU time spent in comet::execution::metrics::utils::update_comet_metric. This function appears to be called whenever native execution wakes up in the polling loop, typically to produce a batch. Starting from the root of the plan, the preorder traversal behavior is:

• For every metric in the node:
  • JNI call to allocate a string for the metric's name
  • JNI call to update the metric using the string as the key
• For every child in the node:
  • JNI call to fetch child metric node
  • Recursively call update_comet_metric

Describe the potential solution

There are a few things to explore:

1. Does reducing the granularity of metrics updates affect the correctness of these metrics? If not, we could update metrics less frequently.
2. Can we eliminate the overhead of repeatedly allocating strings via JNI for every metric? Addressed in perf: Cache jstrings during metrics collection #1029.
3. Can we update an entire node's metrics with a single JNI call, rather than a JNI call for each metric?

Additional context

No response