unable to fetch azure resource's metrics data - azure

Question

I have an azure function .net core 3.1 where I am trying to fetch metrics data
if I use a simpler code
string queryString = "name.value eq 'CpuPercentage'";
ODataQuery<MetadataValue> odataQuery = new ODataQuery<MetadataValue>(queryString);
string resId = app.Id;
var vals = await monitoringClient.Metrics.ListAsync(resourceUri: "/subscriptions/xxxxx/resourceGroups/xxxx/providers/Microsoft.Web/sites/xxxxxx", odataQuery: odataQuery, cancellationToken: CancellationToken.None);
i get the error "resource not found" however am able to see the resource on portal and metrics data is also available on the portal . I registered an app in AD and gave it a readers access on subscription.
when i use
var metricNames = "name.value eq 'CpuPercentage'";
// Time grain is optional when metricNames is present
string timeGrain = "timeGrain eq duration'PT5M'";
//string endDate = string.Format(" and endTime eq {0}", DateTime.Now.ToString("o"));
string startDate = string.Format(" and startTime eq {0}", DateTime.Now.AddHours(-3).ToString("o")).Replace("+", "%2b");
string endDate = string.Format(" and endTime eq {0}", DateTime.Now.ToString("o")).Replace("+", "%2b");
var odataFilterMetrics = new ODataQuery<MetadataValue>(
string.Format(
"{0}{1}{2}{3}",
metricNames,
timeGrain,
startDate,
endDate));
var metrics = await monitoringClient.Metrics.ListAsync(resourceUri: app.Id, odataQuery: odataFilterMetrics, cancellationToken: CancellationToken.None);
i get
only supports conditions of the form ' eq ', ' ne ' and ' sw ': name eq 'CpuPercentage'timeGrain eq duration'PT5M'
i have also tried various dateformats
// Defaulting to 3 hours before the time of execution for these datetimes
//string startDate = string.Format(" and startTime eq {0}", DateTime.Now.AddHours(-3).ToString("o"));
//string endDate = string.Format(" and endTime eq {0}", DateTime.Now.ToString("o"));
string startDate = string.Format(" and startTime eq {0}", DateTime.Now.AddHours(-3).ToString("o")).Replace("+", "%2b");
string endDate = string.Format(" and endTime eq {0}", DateTime.Now.ToString("o")).Replace("+", "%2b");
i am creating the monitoring client like this
private async Task<MonitorManagementClient> GetMonitorClientAsync()
{
var serviceCreds = await ApplicationTokenProvider.LoginSilentAsync("xxx.onmicrosoft.com", "xxx-appId-registered-in-AD", "xx-secret-of-that-app");
var monitorClient = new MonitorManagementClient(serviceCreds);
monitorClient.SubscriptionId = "xxx";
return monitorClient;
}