Fix Sendability warnings

[gjcairo]

This PR fixes Sendability warnings present when enabling strict concurrency checking.

Motivation:

We want to make sure swift-metrics is free of concurrency bugs, and thus removing Sendability warnings is crucial.

Modifications:

• Fixed a couple of Sendability warnings that Xcode would report when enabling strict concurrency checking.
• Also fixed other non-Sendability-related warnings about non-exhaustive switches.

Result:

swift-metrics is now warning-free.

[tomerd]

@swift-server-bot test this please

[ktoso]

In most CI runs this cases the following error:

/code/Sources/Metrics/Metrics.swift:74:9: error: default will never be executed
        @unknown default:
        ^
[8/13] Compiling Metrics Metrics.swift
/code/Sources/Metrics/Metrics.swift:74:9: error: default will never be executed
        @unknown default:
        ^

Can you investigate please @gjcairo ?

[AndrewHoos]

@ktoso Can you give an example of a CI config that causes this? I was about to open a PR to fix this because the enum is not frozen

[ktoso]

We'll have to run tests again to know since it's been so long that since the logs have been removed.

[ktoso]

info here: #137 (comment) @AndrewHoos

[ktoso]

kicking off to answer: #137 (comment)

@swift-server-bot test this please

[ktoso]

• Until 5.5 (excluding 5.5) all builds fail on directly using Sendable which does not exist in these versions.

internal final class AccumulatingMeter: MeterHandler, @unchecked Sendable {
                                                       ^
/code/Sources/CoreMetrics/Locks.swift:47:28: error: cannot find type 'Sendable' in scope
internal final class Lock: Sendable {
                           ^~~~~~~~
[2/8] Compiling CoreMetrics Metrics.swift
/code/Sources/CoreMetrics/Metrics.swift:801:56: error: unknown attribute 'unchecked'
internal final class AccumulatingMeter: MeterHandler, @unchecked Sendable {
                                                       ^
/code/Sources/CoreMetrics/Metrics.swift:801:66: error: cannot find type 'Sendable' in scope
internal final class AccumulatingMeter: MeterHandler, @unchecked Sendable {
                                                                 ^~~~~~~~

• From 5.5 (including 5.5) all builds fail on:

[7/13] Compiling Metrics Metrics.swift
/code/Sources/Metrics/Metrics.swift:74:9: error: default will never be executed
        @unknown default:
        ^
error: fatalError

FYI @gjcairo; cc @AndrewHoos - since you mentioned you'd like to fix things here

[gjcairo]

@swift-server-bot test this please

[FranzBusch]

@swift-server-bot add to allowlist

[FranzBusch]

@swift-server-bot test this please