add ability to encode logs as JSON (in addition to the existing logfmt support)

Author: karlseguin

Makefile

@@ -1,3 +1,4 @@
+F=
 .PHONY: t
 t:
-	zig build test --summary all
+	TEST_FILTER="${F}" zig build test -freference-trace --summary all

build.zig

@@ -12,6 +12,7 @@ pub fn build(b: *std.Build) !void {
 		.root_source_file = .{ .path = "src/logz.zig" },
 		.target = target,
 		.optimize = optimize,
+		.test_runner = "test_runner.zig",
 	});
 	const run_test = b.addRunArtifact(lib_test);
 	run_test.has_side_effects = true;

readme.md

@@ -1,6 +1,6 @@
 Structured Logging for Zig
 
-logz is an opinionated structured logger that outputs to stdout using the logfmt format. It aims to minimize runtime memory allocation by using a pool of pre-allocated loggers. 
+logz is an opinionated structured logger that outputs to stdout using logfmt or JSON. It aims to minimize runtime memory allocation by using a pool of pre-allocated loggers. 
 
 # Installation
 This library supports native Zig module (introduced in 0.11). Add a "logz" dependency to your `build.zig.zon`.
@@ -135,11 +135,13 @@ pub const Config = struct {
 
     // Where to write the output: can be either .stdout or .stderr
     output: Output = .stdout,
+
+    encoding: Encoding = .kv, // or .json
 };
 ```
 
 ### Timestamp and Level
-When using the `debug`, `info`, `warn`, `err` or `fatal` functions, logs will always begin with `@ts=$MILLISECONDS_SINCE_JAN1_1970_UTC @l=$LEVEL`, such as: `@ts=1679473882025 @l=INFO`.
+When using the `debug`, `info`, `warn`, `err` or `fatal` functions, logs will always begin with `@ts=$MILLISECONDS_SINCE_JAN1_1970_UTC @l=$LEVEL`, such as: `@ts=1679473882025 @l=INFO`. With JSON encoding, the object will always have the `"@ts"` and `"@l"` fields.
 
 ### Logger Life cycle
 The logger is implicitly returned to the pool when `log`, `logTo` or `tryLog` is called. In rare cases where `log`, `logTo` or `tryLog` are not called, the logger must be explicitly released using its `release()` function:
@@ -148,6 +150,8 @@ The logger is implicitly returned to the pool when `log`, `logTo` or `tryLog` is
 // This is a contrived example to show explicit release
 var l = logz.info();
 _  = l.string("key", "value");
+
+// actually, on second thought, I don't want to log anything
 l.release();
 ```
 
@@ -205,7 +209,7 @@ pub fn main() !void {
 ### Prefixes
 A pool can be configured with a prefix by setting the `prefix` field of the configuration. When set, all log entries generated by loggers of this pool will contain the prefix. 
 
-The prefix is written as-is, with no escape and no enforcement of being a key=value.
+The prefix is written as-is.
 
 ```zig
 // prefix can be anything []const u8. It doesn't have to be a key=value
@@ -218,8 +222,18 @@ p.info().boolean("tea", true).log();
 
 The above will generate a log line: `keemun @ts=TIMESTAMP @l=INFO tea=Y"`
 
+When using `.json` encoding, your prefix must begin the object:
+
+```zig:
+var p = try logz.Pool.init(allocator, .{.prefix = "=={"});
+defer p.deinit();
+
+p.info().boolean("tea", true).log();
+```
+The above will generate a log line: `=={"@ts":TIMESTAMP, "@l":"INFO", "tea":true}`
+
 ### Multi-Use Logger
-Rather than having a logger automatically returned to the pool when `.log()` or `tryLog()` are called, it is possible to flag the logger for "multi-use". In such cases, the logger must be explicitly returned to the pool using `logger.release()`. This can be enabled by calling `multiuse` on the logger. Important, and the reason this feature exists, is logs by the same logger will share the same attributes up to the point where multiuse was called:
+Rather than having a logger automatically returned to the pool when `.log()` or `tryLog()` are called, it is possible to flag the logger for "multi-use". In such cases, the logger must be explicitly returned to the pool using `logger.release()`. This can be enabled by calling `multiuse` on the logger. Logs created by the logger will share the same attributes up to the point where multiuse was called:
 
 ```zig
 var logger = logz.logger().string("request_id", request_id).multiuse();

src/config.zig

@@ -6,9 +6,15 @@ pub const Config = struct {
 	level: logz.Level = .Info,
 	prefix: ?[]const u8 = null,
 	output: Output = .stdout,
+	encoding: Encoding = .logfmt,
 
 	const Output = enum {
 		stdout,
 		stderr,
 	};
+
+	const Encoding = enum {
+		json,
+		logfmt,
+	};
 };