Don't add "unknown" to Blazor metrics

[JamesNK]

Blazor metrics add "unknown" as a tag value when a value for the tag hasn't been passed in:

aspnetcore/src/Components/Components/src/ComponentsMetrics.cs

Lines 83 to 88 in 8aa92d9

	var tags = new TagList
	{
	{ "aspnetcore.components.type", componentType ?? "unknown" },
	{ "code.function.name", methodName ?? "unknown" },
	{ "aspnetcore.components.attribute.name", attributeName ?? "unknown" }
	};

I don't think this is the right behavior.

All tags don't always need to be present on telemetrymeasurement. It's common for tags to be conditional and only added when needed or a value is known. For example, if a HTTP request doesn't specify a user-agent, no user agent tag is added to the activity.

A more standard behavior is to not include tags when no value is available for it.

For example,

aspnetcore/src/Identity/Core/src/SignInManagerMetrics.cs

Lines 178 to 184 in 8aa92d9

	private static void AddIsPersistent(ref TagList tags, bool? isPersistent)
	{
	if (isPersistent != null)
	{
	tags.Add("aspnetcore.authentication.is_persistent", isPersistent.Value);
	}
	}

[javiercn]

@JamesNK I assume we would want to backport this?

Putting it on RC2 so that we keep track of it.

@pavelsavara co-assigned the issue to you so you keep track

[JamesNK]

@JamesNK I assume we would want to backport this?

Yes

[pavelsavara]

backport #63383