Add transport failure and connect latency counters to node stats

[jbacchus126]

Description

_nodes/stats currently reports transport volume only — rx_count, rx_size_in_bytes, tx_count, tx_size_in_bytes, server_open, and total_outbound_connections. There is no always-available signal for transport failure or connection-open latency, which makes a class of network faults hard to diagnose on a running cluster: the volume counters keep incrementing normally while requests silently time out or connections take seconds to establish.

The connection-open path is the largest gap. A slow or hung connect blocks the cluster applier thread, and no existing metric observes it at all — total_outbound_connections is incremented only after a handshake succeeds, so a node struggling to open connections looks identical to a healthy idle node.

This PR adds six counters to TransportStats:

Field	Meaning
connect_failures	Outbound connection opens that failed or timed out
connect_time_millis	Cumulative time spent on connection opens that succeeded. Average open latency is connect_time_millis / total_outbound_connections
connect_time_millis_max	Worst-case single connection open since node start. Useful for judging whether lowering transport.connect_timeout is safe
channel_close_by_type	Individual channel sockets closed, keyed by channel type (RECOVERY, BULK, REG, STATE, PING, STREAM)
outgoing_timeouts	Outbound requests that received no response within their timeout
requests_failed_on_disconnect	In-flight requests cancelled because the connection closed before a response arrived

channel_close_by_type is the counter that distinguishes partial impairment from a whole-node failure. Because a connection profile assigns separate sockets per channel type, REG/BULK degrading while PING/STATE stay healthy is a materially different fault from a node going away, and today the two are indistinguishable from node stats.

These live in _nodes/stats rather than the telemetry metrics registry so they are available on any cluster with no plugin installed and no configuration, which is what makes them usable during an active incident. Publishing the same values
through MetricsRegistry is a reasonable follow-up, along the lines of NodeRuntimeMetrics (opensearch-project#20844), which exposes JvmStats data as pull-based gauges while _nodes/stats remains the source of truth.

Where the counters are incremented

• TcpTransport.ChannelsConnectedListener — connect latency is measured from listener construction to successful handshake; closeAndFail increments connect_failures. The countDown.fastForward() guard on the failure and timeout paths means a single attempt cannot be counted twice.
• TcpTransport.NodeChannels — a per-channel close listener attributes socket closes to the channel types sharing that socket. The reverse index is derived from ConnectionProfile.ConnectionTypeHandle's own offset/length range, so attribution matches the channel that getChannel actually selects.
• TransportService.onConnectionClosed — counts pruned in-flight handlers.
• TransportService.TimeoutHandler — counts expired requests.

None of the counters sit on the per-request send path, so there is no added contention at request volume.

DEBUG logging

Also adds DEBUG logging for connect latency, send failures, and per-channel socket closes. NodeConnectionsService now reports elapsed time on both successful and failed connects, so failed to connect to {} after [Nms] distinguishes an
immediate refusal from a full connect-timeout stall. Nothing is added at INFO, so production log volume is unchanged.

Sample output

GET _nodes/stats/transport:

"transport": {
  "server_open": 26,
  "total_outbound_connections": 143,
  "rx_count": 1043321,
  "rx_size_in_bytes": 884736102,
  "tx_count": 1043298,
  "tx_size_in_bytes": 901324887,
  "channel_close_by_type": {
    "RECOVERY": 0,
    "BULK": 12,
    "REG": 12,
    "STATE": 0,
    "PING": 0,
    "STREAM": 0
  },
  "outgoing_timeouts": 37,
  "requests_failed_on_disconnect": 214,
  "connect_failures": 6,
  "connect_time_millis": 8412,
  "connect_time_millis_max": 4903
}

Backwards compatibility

New fields are written and read only when the negotiated stream version is 3.8.0 or later, so mixed-version clusters and cross-cluster connections to older clusters are unaffected — older nodes exchange only the pre-existing counters and report the new ones as zero.

This is purely additive to the _nodes/stats response; no existing field changes name, type, or meaning. An empty channel_close_by_type is omitted rather than rendered as an empty object, so stats built without it are byte-identical to the previous response shape.

TransportStats also gains a Builder(TransportStats) copy constructor.
TransportService.stats() previously restated all twelve fields by