Support registry as Pushgateway second argument

[bugprone]

Purpose

Pushgateway accepts an optional options argument before the registry argument. In TypeScript, new Pushgateway(url, registry) was accepted because options was typed as any, but at runtime that registry object was treated as request options and the Pushgateway fell back to the global registry.

This makes the two-argument registry form work at runtime and declares it explicitly in the TypeScript definitions.

Fixes #652.

Changes

• Treat a Registry passed as the second Pushgateway constructor argument as the registry
• Add constructor overloads for new Pushgateway(url, registry) and new Pushgateway(url, options, registry)
• Add a small Pushgateway.Options type with requireJobName plus request option passthrough
• Add runtime coverage for pushing metrics from a second-argument registry
• Add TypeScript compile coverage for the supported constructor forms
• Update the unreleased changelog

Testing

• npm test -- --runInBand --forceExit
• Manual smoke check with a local HTTP server verifying new Pushgateway(url, registry).push() sends metrics from the provided registry

[jdmarshall]

Related to #652

[jdmarshall]

LGTM