Make metrics less error prone

[Half-Shot]

The bridge is prone to erroring with things like:

(node:1) UnhandledPromiseRejectionWarning: TypeError: Cannot read property 'get' of undefined
    at PrometheusBridgeMetrics.sdkEndMetric (/src/metrics.ts:207:54)
    at /node_modules/matrix-bot-sdk/lib/metrics/Metrics.js:73:39
    at Array.forEach (<anonymous>)
    at Metrics.end (/node_modules/matrix-bot-sdk/lib/metrics/Metrics.js:73:24)
    at /node_modules/matrix-bot-sdk/lib/metrics/decorators.js:35:40
    at <anonymous>
    at processTicksAndRejections (internal/process/task_queues.js:95:5)

Which is mostly due to the fact that our metrics system is:

1. Not correctly initalising the Maps
2. Listening in the wrong places for the wrong metrics.

I've also updated the matrix-appservice-bridge library to be on the latest version, I don't think it's changes will affect us too badly.

[Half-Shot]

Depends on #811 due to requiring Node 14.

[AndrewFerr]

Maybe let's just make a single change to update all of these at once 😆

[AndrewFerr]

This should trigger on SIGTERM too.

[AndrewFerr]

Nitpick suggestion:

if (this._bot) {
    this._bot.destroy();
    this._bot = undefined; 
}

since there's a branch buried in the ?. anyways.