feat(telemetry): add batch-size and discovered-path histograms

Review feedback asked for a Histogram rather than a span attribute at these two
places. Both, not either: the attribute answers how big one sampled request was,
which an aggregate cannot, and the histogram answers the distribution across all
requests, which an unsampled trace never reveals. Both attributes stay.

The metrics land here rather than with the attributes because neither
HistogramBuckets.h nor the metric macro exists on the branch that added them.

Both use kObjectCountBuckets. The argument is the floor, not the ceiling: the SDK
default edges start 0,5,10,25, so an ordinary batch of one to five falls in a
single bucket and every quantile becomes an interpolation on one edge. The object
ladder puts five edges over the mass of both distributions. Path count is bounded
at 352 by kMaxPaths times kMaxAutoSrcCur and cannot saturate. Batch size can, at
roughly 333k, but no measured traffic goes near it, so the ladder is not widened
for a range nothing occupies; the runbook records the overflow query and a test
asserts it stays readable.

.github/scripts/levelization/results/ordering.txt

@@ -193,6 +193,7 @@ tests.libxrpl > xrpl.config
 tests.libxrpl > xrpl.consensus
 tests.libxrpl > xrpl.core
 tests.libxrpl > xrpld.app
+tests.libxrpl > xrpld.rpc
 tests.libxrpl > xrpld.telemetry
 tests.libxrpl > xrpl.json
 tests.libxrpl > xrpl.ledger
@@ -313,6 +314,7 @@ xrpld.rpc > xrpl.basics
 xrpld.rpc > xrpl.config
 xrpld.rpc > xrpl.core
 xrpld.rpc > xrpld.core
+xrpld.rpc > xrpld.telemetry
 xrpld.rpc > xrpl.json
 xrpld.rpc > xrpl.ledger
 xrpld.rpc > xrpl.net