Update custom metric in AspNetCore example

[danelson]

open-telemetry/opentelemetry-dotnet#4107 updated the AspNetCore example to include custom metrics and activities. There was some discussion related to the counter that was added. The current implementation shows the correct pattern for meter/instrument creation, but it does not do a great job of providing a meaningful metric.

Ideally the metric would be reworked so that it is:

• Unique - Provides meaning outside of something like the number of times the endpoint was executed which can be determined from metrics being created by AddAspNetCoreInstrumentation
• Contextual - Provides meaning in the context of the work being performed (generating a forecast)

Suggestions include:

• Adding a dimension to the metric
• Retrieving forecasts from a cache and measuring cache hits/misses
• Return variable length forecasts and measure those that exceed a certain number of days (simulating expensive operations)

Notes:

• The custom metric does not need to be a counter