Summary#observe does not throw when adding additional labels to labelset

[samt]

Reproduction:

const { Summary } = require('prom-client');

const summary = new Summary({
    name: 'foobar',
    help: 'Foo Bar',
    labelNames: ['foo']
});

summary.observe({ foo: 'bar', baz: 'qaz' }, 10);
console.log('did NOT throw');

Expected

Error: Added label "baz" is not included in initial labelset: [ 'foo' ]
    at Object.keys.forEach.label (/Users/samt/Repos/prom-client/lib/validation.js:26:10)
    at Array.forEach (native)
    at validateLabel (/Users/samt/Repos/prom-client/lib/validation.js:24:22)
    at /Users/samt/Repos/prom-client/lib/gauge.js:238:3
    at Gauge.set (/Users/samt/Repos/prom-client/lib/gauge.js:91:32)
    at Object.<anonymous> (/Users/samt/Repos/reprod-prom/index.js:19:7)
    at Module._compile (module.js:570:32)
    at Object.Module._extensions..js (module.js:579:10)
    at Module.load (module.js:487:32)
    at tryModuleLoad (module.js:446:12)

Actual

did NOT throw

---

Issue is caused by lib/summary.js#L245

validateLabel(this.labelNames, this.labels);

Should be:

validateLabel(this.labelNames, labels);