fix(registry): default to summation for metrics without an aggregator

[chuanghiduoc]

Problem

Both the README ("Custom metrics are summed across workers by default") and the worker/cluster JSDoc ("or by summation if aggregator is undefined") promise a 'sum' fallback for metrics without an explicit aggregator. Two recent changes broke that contract:

1. Registry.aggregate() throws 'undefined' is not a defined aggregator. when any snapshot lacks the property — snapshots produced by registerMetric() with a plain object, or hand-written wrappers. This crashes clusterMetrics() / workerMetrics() entirely.
2. getMetricsAsArray('sum') silently filters those metrics out (aggregator === metric.aggregator is false when the property is missing), so their values are lost on the shutdown-flush path added for workers in ef1cea2.

Reproduction on current main:

const { Registry } = require('prom-client');
Registry.aggregate([
  [{ help: 'h', name: 'm', type: 'gauge', values: [{ value: 5, labels: {} }] }],
]);
// Error: 'undefined' is not a defined aggregator.

and:

const register = new Registry();
new Counter({ name: 'normal', help: 'h', registers: [register] }); // aggregator defaults to 'sum'
register.getMetricsAsArray('sum').length; // 0 if the metric object lacks .aggregator

Note that new Counter({...}) sets aggregator: 'sum' via the Metric constructor default, so built-in metric classes mask the issue; it only surfaces with wrapper/plain-object metrics, which is exactly what the aggregation and shutdown paths produce internally.

Fix

Restore the documented 'sum' fallback in both places (metric.aggregator ?? 'sum').

Testing

• two new tests in test/registerTest.js: aggregate() sums snapshots without an aggregator; getMetricsAsArray('sum') / getMetricsAsJSON('sum') include them
• full unit suite: 29 suites / 581 tests passing

[jdmarshall]

Hmmm.. I had code for this but then convinced myself it wasn't needed. :/