DocDB: Add the lag column to list_all_masters yb-admin output

[keisku]

Closes #28675

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There is a big number on in the Heartbeat Lag clumn (this PR added) since yb-master on the node3 stopped.

./bin/yb-ctl status
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| Node Count: 3 | Replication Factor: 3                                                            |
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| JDBC                : jdbc:postgresql://127.0.0.1:5433/yugabyte                                  |
| YSQL Shell          : bin/ysqlsh                                                                 |
| YCQL Shell          : bin/ycqlsh                                                                 |
| YEDIS Shell         : bin/redis-cli                                                              |
| Web UI              : http://127.0.0.1:7000/                                                     |
| Cluster Data        : /Users/keisukeumegaki/yugabyte-data                                        |
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| Node 1: yb-tserver (pid 93159), yb-master (pid 58555)                                            |
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| JDBC                : jdbc:postgresql://127.0.0.1:5433/yugabyte                                  |
| YSQL Shell          : bin/ysqlsh                                                                 |
| YCQL Shell          : bin/ycqlsh                                                                 |
| YEDIS Shell         : bin/redis-cli                                                              |
| data-dir[0]         : /Users/keisukeumegaki/yugabyte-data/node-1/disk-1/yb-data                  |
| yb-tserver Logs     : /Users/keisukeumegaki/yugabyte-data/node-1/disk-1/yb-data/tserver/logs     |
| yb-master Logs      : /Users/keisukeumegaki/yugabyte-data/node-1/disk-1/yb-data/master/logs      |
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| Node 2: yb-tserver (Stopped), yb-master (pid 91341)                                              |
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| JDBC                : jdbc:postgresql://127.0.0.2:5433/yugabyte                                  |
| YSQL Shell          : bin/ysqlsh -h 127.0.0.2                                                    |
| YCQL Shell          : bin/ycqlsh 127.0.0.2                                                       |
| YEDIS Shell         : bin/redis-cli -h 127.0.0.2                                                 |
| data-dir[0]         : /Users/keisukeumegaki/yugabyte-data/node-2/disk-1/yb-data                  |
| yb-tserver Logs     : /Users/keisukeumegaki/yugabyte-data/node-2/disk-1/yb-data/tserver/logs     |
| yb-master Logs      : /Users/keisukeumegaki/yugabyte-data/node-2/disk-1/yb-data/master/logs      |
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| Node 3: yb-tserver (pid 9970), yb-master (Stopped)                                               |
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| JDBC                : jdbc:postgresql://127.0.0.3:5433/yugabyte                                  |
| YSQL Shell          : bin/ysqlsh -h 127.0.0.3                                                    |
| YCQL Shell          : bin/ycqlsh 127.0.0.3                                                       |
| YEDIS Shell         : bin/redis-cli -h 127.0.0.3                                                 |
| data-dir[0]         : /Users/keisukeumegaki/yugabyte-data/node-3/disk-1/yb-data                  |
| yb-tserver Logs     : /Users/keisukeumegaki/yugabyte-data/node-3/disk-1/yb-data/tserver/logs     |
| yb-master Logs      : /Users/keisukeumegaki/yugabyte-data/node-3/disk-1/yb-data/master/logs      |
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./build/latest/bin/yb-admin --master_addresses 127.0.0.1:7100,127.0.0.2:7100,127.0.0.3:7100 list_all_masters
Master UUID                             RPC Host/Port           State           Role            Broadcast Host/Port     Heartbeat Lag
9c26c847c2c24691a4ef839ccbb82c15        127.0.0.1:7100          ALIVE           LEADER          N/A                     0ms
577f32bb523847249e338b3f89981e1a        127.0.0.2:7100          ALIVE           FOLLOWER        N/A                     178ms
8214242e58ac478595d15d9f5244dc6a        127.0.0.3:7100          NETWORK_ERROR   UNKNOWN         N/A                     678092ms

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Note

Low Risk
Change is limited to CLI output formatting and best-effort RPC data retrieval; primary risk is minor output/compatibility impact for scripts parsing list_all_masters.

Overview
Adds a Heartbeat Lag column to yb-admin list_all_masters output by best-effort querying the leader via GetMasterHeartbeatDelays, showing 0ms for the leader, per-follower lag in ms when available, and N/A otherwise.

Updates multi-master integration tests to accommodate the new non-deterministic lag values by normalizing \d+ms before comparing outputs, and adds a new test asserting the column/header and basic lag formatting are present.

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[cursor]

Test assumes followers always have lag data

Medium Severity

The test requires all follower lines to contain ms, but the implementation outputs N/A for followers when has_lag_data is true but their UUID isn't found in the heartbeat map (line 1509 of yb-admin_client.cc). A follower missing from the heartbeat response would fail this assertion despite being valid output from the implementation.