This series closes a gap in how CQL request and response sizes are reported.
Previously, request_size and response_size were tracked as simple counters,
providing only cumulative totals per shard. This made it difficult to understand
the distribution of message sizes and identify potential issues with very large
or very small requests.
After this series, the CQL transport reports detailed histogram metrics showing
the distribution of request and response sizes. These histograms are tracked
per-instance, per-type (per ops), and per-scheduling-group, providing
much better visibility into CQL traffic patterns.
The histograms are collected for QUERY, EXECUTE, and BATCH operations, which are
the primary data path operations where message size distribution is most relevant.
This data can help identify:
- Clients sending unexpectedly large requests
- Operations with oversized result sets
- Scheduling group differences in traffic patterns
To support this, the series extends the approx_exponential_histogram template to
handle accurate sum, adds a bytes_histogram type alias optimized for byte-range measurements (1KB to 1GB).
The existing per-shard counter metrics are maintained for backward compatibility.
Metrics example:
```
scylla_transport_cql_request_bytes{kind="BATCH",scheduling_group_name="sl:default",shard="0"} 129808
scylla_transport_cql_request_bytes{kind="EXECUTE",scheduling_group_name="sl:default",shard="0"} 227409
scylla_transport_cql_request_bytes{kind="PREPARE",scheduling_group_name="sl:default",shard="0"} 631
scylla_transport_cql_request_bytes{kind="QUERY",scheduling_group_name="sl:default",shard="0"} 2809
scylla_transport_cql_request_bytes{kind="QUERY",scheduling_group_name="sl:driver",shard="0"} 4079
scylla_transport_cql_request_bytes{kind="REGISTER",scheduling_group_name="sl:default",shard="0"} 98
scylla_transport_cql_request_bytes{kind="STARTUP",scheduling_group_name="sl:driver",shard="0"} 432
scylla_transport_cql_request_histogram_bytes_sum{kind="QUERY",scheduling_group_name="sl:driver"} 4079
scylla_transport_cql_request_histogram_bytes_count{kind="QUERY",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="1024.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="2048.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="4096.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="8192.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="16384.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="32768.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="65536.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="131072.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="262144.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="524288.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="1048576.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="2097152.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="4194304.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="8388608.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="16777216.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="33554432.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="67108864.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="134217728.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="268435456.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="536870912.000000",scheduling_group_name="sl:driver"} 57
scylla_transport_cql_request_histogram_bytes_bucket{kind="QUERY",le="1073741824.000000",scheduling_group_name="sl:driver"} 57
```
**The field sees it as an important issue**
Fixes#14850Closesscylladb/scylladb#28419
* github.com:scylladb/scylladb:
test/boost/estimated_histogram_test.cc: Switch to real Sum
transport/server: to bytes_histogram
approx_exponential_histogram: Add sum() method for accurate value tracking
utils/estimated_histogram.hh: Add bytes_histogram
99244179f7