[HttpClient & ASP.NET Core] Document request durations

[vishweshbankwar]

Fixes #3277
Design discussion issue #

Changes

Borrowing the definitions from https://learn.microsoft.com/en-us/dotnet/core/diagnostics/built-in-metrics-system-net#instrument-httpclientrequestduration and https://learn.microsoft.com/en-us/dotnet/core/diagnostics/built-in-metrics-aspnetcore#instrument-httpserverrequestduration as the implementations record the times around the same time when DiagnosticSource events are written.

For ASP.NET Core: Validated the metrics are recorded same time as DiagnosticSource callbacks are fired.
https://github.com/dotnet/aspnetcore/blob/5b1ad1ee4934be7d587e50670524876763cd45eb/src/Hosting/Hosting/src/Internal/HostingApplicationDiagnostics.cs#L156-L211

For HttpClient: Checked the handler configuration https://github.com/dotnet/runtime/blob/49d7bc83d986c32e8678001c841b1b836bc3e428/src/libraries/System.Net.Http/src/System/Net/Http/HttpClientHandler.cs#L40-L45 and also confirmed with @antonfirsov

Merge requirement checklist

• CONTRIBUTING guidelines followed (nullable enabled, static analysis, etc.)
• Unit tests added/updated
• Appropriate CHANGELOG.md files updated for non-trivial changes
• Changes in public API reviewed (if applicable)

[antonfirsov]

Client part looks good.

[TimothyMothra]

Would it be useful to include the links to the learn.microsoft doc that defies the metric?
(ie: for more information see (link)).

[vishweshbankwar]

Would it be useful to include the links to the learn.microsoft doc that defies the metric? (ie: for more information see (link)).

The link to metrics are already included in Readme. https://github.com/open-telemetry/opentelemetry-dotnet/tree/main/src/OpenTelemetry.Instrumentation.AspNetCore#list-of-metrics-produced