Histogram custom tags are not parsed correctly and not showing up.

[XiLiuRoy]

Description

Histogram meter custom tags value are not parsed using correct separater "=", as when sending up Histogram (AggregatePercentilePayload) with custom tags to dotnet monitor, in file MetricStore.cs line, it's missing a case for "AggregatePercentilePayload", which caused the IsMeter method to return false, and using ":" to parse custom tags, as a result, it returns nothing as custom tags.

Configuration

• What OS and version, and what distro if applicable?
  MacOS 14.6.1 (23G93)
• What is the architecture (x64, x86, ARM, ARM64)?
  ARM64
• Do you know whether it is specific to that configuration?
  By looking at the source, I don't think it's specific to certain configurations.
• Are you running in any particular type of environment? (e.g. Containers, a cloud scenario, app you are trying to target is a different user)
  No
• Is it a self-contained published application?
  No
• What's the output of dotnet info
  `.NET SDK:
  Version: 8.0.203
  Commit: 5e1ceea679
  Workload version: 8.0.200-manifests.205a5945

Runtime Environment:
OS Name: Mac OS X
OS Version: 14.6
OS Platform: Darwin
RID: osx-arm64
Base Path: /usr/local/share/dotnet/sdk/8.0.203/

.NET workloads installed:
There are no installed workloads to display.

Host:
Version: 9.0.0-rc.2.24473.5
Architecture: arm64
Commit: 990ebf52fc

.NET SDKs installed:
6.0.418 [/usr/local/share/dotnet/sdk]
7.0.405 [/usr/local/share/dotnet/sdk]
8.0.203 [/usr/local/share/dotnet/sdk]

.NET runtimes installed:
Microsoft.AspNetCore.App 6.0.26 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 7.0.15 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 8.0.3 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.AspNetCore.App 9.0.0-rc.2.24474.3 [/usr/local/share/dotnet/shared/Microsoft.AspNetCore.App]
Microsoft.NETCore.App 6.0.26 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 7.0.15 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 8.0.3 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]
Microsoft.NETCore.App 9.0.0-rc.2.24473.5 [/usr/local/share/dotnet/shared/Microsoft.NETCore.App]

Other architectures found:
None

Environment variables:
Not set

global.json file:
Not found

Learn more:
https://aka.ms/dotnet/info

Download .NET:
https://aka.ms/dotnet/download
`

Regression?

No, from the source code, this may never worked before for Histogram custom tags.

Other information

It seems the issue is in MetricStore.cs => IsMeter method, and adding additional case for AggregatePercentilePayload seems to do the trick.

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