refactor: Make ResultsFulfillerMetrics injectable for test isolation

[georgi]

Convert ResultsFulfillerMetrics from an object to a class that accepts a Meter parameter. This allows tests to inject their own meter instance for better test isolation and prevents test pollution from shared global state.

• Convert ResultsFulfillerMetrics from object to class
• Add create() factory method for default Instrumentation meter
• Add metrics parameter to ResultsFulfiller constructor
• Update ResultsFulfillerAppRunner to create metrics instance

[wfa-reviewable]

This change is 

[kungfucraig]

@kungfucraig reviewed 3 of 3 files at r1, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @georgi and @Marco-Premier)

---

src/main/kotlin/org/wfanet/measurement/edpaggregator/resultsfulfiller/ResultsFulfiller.kt line 106 at r1 (raw file):

  private val fulfillerSelector: FulfillerSelector,
  private val responsePageSize: Int? = null,
  private val metrics: ResultsFulfillerMetrics = ResultsFulfillerMetrics.create(),

I'd prefer if we didn't default this. It causes long-term maintenance headaches.

[georgi]

@georgi reviewed 1 of 3 files at r1, 4 of 4 files at r2, all commit messages.
Reviewable status: all files reviewed, 1 unresolved discussion (waiting on @kungfucraig and @Marco-Premier)

---

src/main/kotlin/org/wfanet/measurement/edpaggregator/resultsfulfiller/ResultsFulfiller.kt line 106 at r1 (raw file):

Previously, kungfucraig (Craig Wright) wrote…

I'd prefer if we didn't default this. It causes long-term maintenance headaches.

Done.

[kungfucraig]

@kungfucraig reviewed 6 files and all commit messages, and resolved 1 discussion.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on @Marco-Premier).