Add possibility to use google credentials file from file referenced in ENV variable

src/Serilog.Sinks.GoogleCloudLogging/GoogleCloudLoggingSink.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using System.IO;
 using System.Threading;
@@ -57,9 +57,11 @@ public GoogleCloudLoggingSink(GoogleCloudLoggingSinkOptions sinkOptions, ITextFo
         }
 
         // logging client for google cloud apis
-        _client = _sinkOptions.GoogleCredentialJson.IsNullOrWhiteSpace()
+        var clientBuilder = CreateBuilderBasedOnCredentials(_sinkOptions);
+
+        _client = clientBuilder == null
             ? LoggingServiceV2Client.Create()
-            : new LoggingServiceV2ClientBuilder { JsonCredentials = _sinkOptions.GoogleCredentialJson }.Build();
+            : clientBuilder.Build();
     }
 
     public Task EmitBatchAsync(IEnumerable<LogEvent> events)
@@ -152,5 +154,26 @@ private static string GetLogEntryMessage(LogEntry logEntry)
         _ => LogSeverity.Default
     };
 
+    private static LoggingServiceV2ClientBuilder? CreateBuilderBasedOnCredentials(GoogleCloudLoggingSinkOptions sinkOptions)
+    {
+        string? credentialsFilePath = null;
+        if (!sinkOptions.GoogleCredentialsEnvironmentVariableName.IsNullOrWhiteSpace())
+        {
+            credentialsFilePath = Environment.GetEnvironmentVariable(sinkOptions.GoogleCredentialsEnvironmentVariableName);
+        }
+
+        LoggingServiceV2ClientBuilder? clientBuilder = null;
+        if (!sinkOptions.GoogleCredentialJson.IsNullOrWhiteSpace())
+        {
+            clientBuilder = new LoggingServiceV2ClientBuilder { JsonCredentials = sinkOptions.GoogleCredentialJson };
+        }
+        else if (!credentialsFilePath.IsNullOrWhiteSpace())
+        {
+            clientBuilder = new LoggingServiceV2ClientBuilder { CredentialsPath = credentialsFilePath };
+        }
+
+        return clientBuilder;
+    }
+
     public Task OnEmptyBatchAsync() => Task.CompletedTask;
 }

src/Serilog.Sinks.GoogleCloudLogging/GoogleCloudLoggingSinkExtensions.cs

@@ -1,4 +1,4 @@
-using System;
+using System;
 using System.Collections.Generic;
 using Serilog.Configuration;
 using Serilog.Core;
@@ -67,6 +67,7 @@ public static LoggerConfiguration GoogleCloudLogging(
         bool useSourceContextAsLogName = true,
         bool useLogCorrelation = true,
         string? googleCredentialJson = null,
+        string? googleCredentialsEnvironmentVariableName = null,
         string? serviceName = null,
         string? serviceVersion = null,
         int? batchSizeLimit = null,
@@ -87,7 +88,8 @@ public static LoggerConfiguration GoogleCloudLogging(
             useLogCorrelation,
             googleCredentialJson,
             serviceName,
-            serviceVersion
+            serviceVersion,
+            googleCredentialsEnvironmentVariableName
         );
 
         return loggerConfiguration.GoogleCloudLogging(

src/Serilog.Sinks.GoogleCloudLogging/GoogleCloudLoggingSinkOptions.cs

@@ -1,4 +1,4 @@
-using System.Collections.Generic;
+using System.Collections.Generic;
 
 namespace Serilog.Sinks.GoogleCloudLogging;
 
@@ -50,6 +50,11 @@ public class GoogleCloudLoggingSinkOptions
     /// </summary>
     public string? GoogleCredentialJson { get; set; }
 
+    /// <summary>
+    /// File path with Content of Google Cloud JSON credentials to override using Application Default credentials.
+    /// </summary>
+    public string? GoogleCredentialsEnvironmentVariableName { get; set; }
+
     /// <summary>
     /// Attach service name to log entries (added as `serviceContext.service` metadata in `jsonPayload`).
     /// Required for logged exceptions to be forwarded to StackDriver Error Reporting.
@@ -115,7 +120,8 @@ public GoogleCloudLoggingSinkOptions(
         bool useLogCorrelation = true,
         string? googleCredentialJson = null,
         string? serviceName = null,
-        string? serviceVersion = null)
+        string? serviceVersion = null,
+        string? googleCredentialsEnvironmentVariableName = null)
     {
         ProjectId = projectId;
         ResourceType = resourceType;
@@ -134,5 +140,6 @@ public GoogleCloudLoggingSinkOptions(
         GoogleCredentialJson = googleCredentialJson;
         ServiceName = serviceName;
         ServiceVersion = serviceVersion;
+        GoogleCredentialsEnvironmentVariableName = googleCredentialsEnvironmentVariableName;
     }
 }