Repeating calls on addTaggedMetrics() doesn't return existing metrics

[chengB12]

If we call addTaggedMetrics() with same name/tags, but different counter instances,
registry.addTaggedMetric will pass without error, however, the second call actually has no effect
Proof:

it('test behavior of wavefront metrics', () => {
        const registry: wfMetrics.Registry = new wfMetrics.Registry({})
        const tag1 = new wfMetrics.Counter()
        registry.addTaggedMetric('test.metrics.counter.tagged', tag1, { tag: '1' })
        const tag2 = new wfMetrics.Counter()
        registry.addTaggedMetric('test.metrics.counter.tagged', tag2, { tag: '1' })
        const realCounter = registry.getMetric('test.metrics.counter.tagged-tags=[["tag","1"]]')
        expect(realCounter).eq(tag1)
})

Rational: we have dynamically generated tags, and we don't know if a counter of same key/tags already exists unless we keep in-memory track of metrics we created, naturally, we call addTaggedMetrics() multiple times, and expect to having original counter.

solution:
make addTaggedMetrics() return existing metric if there is already one in registry,