Switch to multi-dimensional metrics

As of today, Azure Functions provide an individual metric per function for the same scope such as `{function name} AvgDurationMs`.


This means that if we have 100 functions, we'll have n * 100 metrics where n is the number of different metrics Azure Functions provides.

### Describe the solution you'd like
Would be great if Azure Functions runtime would move to **multi-dimensional metrics in Azure Monitor** and label every metric measurement with the name of the function.

This would give the following benefits:
- **Reduce the number of metrics** to n, regardless of the amount of functions.
- Allow metric consumers to **[filter & split the metric](https://docs.microsoft.com/en-gb/azure/azure-monitor/platform/metrics-charts#how-do-i-create-a-new-chart)** and more **easily create dashboards**
- Create **one alert on a given metric for all functions**

All kudos go to @jcorioland

Start by reviewing how the Azure Functions runtime currently emits per-function metrics and how Azure Monitor supports multi-dimensional metrics. Done means metrics are emitted with the function name as a dimension, the metric count no longer grows per function, and consumers can filter, split, dashboard, and alert across functions.

Written by the indexing model from the issue text.