Make TestMetrics.FullKey tolerate repeated dimension names - #244

TestMetrics traps when a metric is created with a repeated dimension name.

Motivation:

TestMetrics.FullKey hashes and compares its dimensions through Dictionary(uniqueKeysWithValues:), which traps on a repeated name:

let metrics = TestMetrics()
Counter(label: "requests", dimensions: [("tag", "a"), ("tag", "b")], factory: metrics)
// Fatal error: Duplicate values for key: 'tag'

The metrics API accepts that input and every other factory carries it through.

Modifications:

hash(into:) and == read the dimensions sorted by name then value: identity stays order-insensitive, both read the same representation, and repeated names are kept. One test, which crashes without the fix.

Result:

TestMetrics records the instrument instead of killing the test process.