Change aggregation job size metric

The previous sizes were mostly useless for monitoring as they included
amounts of reports that were no where near close to being supported,
these sizes should let us distinguish between very small jobs, medium
size jobs and jobs stretching the capacity of the system.

daphne/src/metrics.rs

@@ -54,8 +54,9 @@ impl DaphneMetrics {
             format!("{front}aggregation_job_batch_size"),
             "Number of records in an incoming AggregationJobInitReq.",
             &["host"],
-            exponential_buckets(5.0, 3.0, 6)
-                .expect("this shouldn't panic for these hardcoded values"), // <250, <500, ... <1500, +Inf
+            // <1, <2, <4, <8, ... <256, +Inf
+            exponential_buckets(1.0, 2.0, 8)
+                .expect("this shouldn't panic for these hardcoded values"),
             registry
         )
         .map_err(|e| fatal_error!(err = ?e, "failed to register aggregation_job_batch_size"))?;