Fix label equality issue

Update minimum required Swift version to 5.2 (#51)

Motivation:

- SwiftNIO will soon raise it's minimum supported version of Swift from
  5.0 to 5.2 (see: apple/swift-nio#1860)
- Swift 5.0 and 5.1 users will not be able to use future versions of NIO
  (most likely from version 2.30.0).
- Since Prometheus depends on NIO and NIO is so pervasive in the
  ecosystem it makes sense to follow NIOs lead in terms of supported
  Swift versions.

Modifications:

- Update Package.swift to change the tools version to 5.2
- Use the newer syntax for specifying target dependencies
- Remove Package.resolved since it has little value for libraries
- Update circle.yml to update CI jobs
- Update Swift version badge in README.md

Co-authored-by: Jari (LotU) <j.koopman@jarict.nl>

Perf improvements (#50)

Motivation:

Metrics collecting systems should be efficient enough so that their
impact on the system being instrumented is negligible.

Modifications:

- Store `PromMetric`s in a dictionary keyed by label in
  `PrometheusClient`; this gives fast lookup when checking if a metric
  already exists
- Remove the `metricTypeMap`, we can recover the same information from
  `PromMetric`
- Remove calls to `getMetricInstrumet` in `PrometheusMetricsFactory`
  these were redundant as the same check is done in `createCounter` etc.
- In each `createCounter` (etc.) call we now hold the lock for the
  duration of the call to avoid races between checking the cache and
  creating and storing a new metric.
- The `PrometheusLabelSanitizer` now checks whether input needs
  santizing instead of santizing all input
- Sanitizing is done in one step, mapping each utf8 code point to a
  sanitized code point rather than lowercasing and then checking the
  validity of each character.

Result:

- Sanitizing pre-sanitized labels is ~100x faster
- Sanitizing non-sanitized label is ~20x faster
- Incrementing 10 counters is ~20x faster
- Incrementing 100 counters is ~40x faster
- Incrementing 1000 counters is ~250x faster
- (Similar results for other metrics.)

Add fix and test for MetricsSystem.prometheus() (#49)

Histogram backed timer (#46)

* implement histogram backed timer

* hide timer implementation enum; namespace PrometheusMetricsFactory classes

* address review

* fix test testHistogramBackedTimer_scaleFromNanoseconds

* address PR review comments

fix Buckets helpers to not throw fatal error (#41)

* fix Buckets to not throw fatal error

* fix compiler warning

* simplify linear buckets creation

Co-authored-by: Jari (LotU) <j.koopman@jarict.nl>

Remove CoreGraphics (#35)

* -remove CoreGraphics

* Update Utils.swift

Prevent the compiler finding the wrong CGFloat (#34)

Add LabelSanitizers (#32)

* Add s

* Add docs

* Update Sources/Prometheus/PrometheusMetrics.swift

Co-Authored-By: Konrad `ktoso` Malawski <konrad_malawski@apple.com>

* Remarks

Co-authored-by: Konrad `ktoso` Malawski <konrad_malawski@apple.com>

Update SwiftPrometheus to use SwiftMetric 2.* (TimeUnit change to str…

…uct) (#31)

* Update SwiftPrometheus to use SwiftMetric 2.* SemVer (TimeUnit def change)

* Update Package.swift

* Update SummaryTests.swift

* Update Summary.swift