Statistics analogues to node.js agent

[mnutt]

In switching from the standard node.js agent to undici, it looks like undici will offer more granular statistics than the node.js agent which is great but I'm initially hoping to replicate what we pull from the node.js agent to send to prometheus: the length of sockets, freeSockets, and requests. I wanted to confirm if these were equivalents (or if there even are equivalents):

// node agent
Object.values(agent.sockets)[0] || []).length
// undici
balancedPool[kClients].reduce((total, pool) => total + pool[kClients].filter(c => c[kConnected]).length, 0)

// node agent
Object.values(agent.freeSockets)[0] || []).length
// undici
balancedPool[kClients].reduce((total, pool) => total + pool[kClients].filter(c => c[kConnected] && !c[kNeedDrain]).length, 0)

// node agent
Object.values(agent.requests)[0] || []).length
balancedPool[kClients].reduce((total, pool) => total + pool[kClients].filter(c => c[kRunning]).length, 0)

[mcollina]

Would you like to send a PR to add those as public properties? You should not really access internal fields.