add support for OpenMetrics message format

This commit adds support for the OpenMetrics message format[1] which is
equivalent to the message format of Prometheus. Its advantage is to be
easily human readable as well as consumable by time series databases.

Lastly it's possible to display results directly within the GitLab CI.

It's now possible to pass 'openmetrics' as a message format, an example
is shown below:

criterion_benchmark_result_ns{id="address_scan",confidence="estimate",input_size="4",aggregation="slope"} 33811.02693018697
criterion_benchmark_result_ns{id="address_scan",confidence="upper_bound",input_size="4",aggregation="slope"} 35170.884432780265
criterion_benchmark_result_ns{id="address_scan",confidence="lower_bound",input_size="4",aggregation="slope"} 32402.287415202973
criterion_benchmark_info{id="address_scan",input_size="4",report_directory="/path/criterion/reports/address_scan/4"} 1

[1]: https://openmetrics.io

Signed-off-by: Kevin Becker <[email protected]>
Signed-off-by: Paul Spooren <[email protected]>

CHANGELOG.md

@@ -5,6 +5,8 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/)
 and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.html).
 
 # [Unreleased]
+### Added
+- New message output format for [OpenMetrics](https://openmetrics.io).
 
 ## [1.1.0] - 2021-07-28
 ### Fixed
@@ -66,4 +68,4 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 [1.0.0]: https://github.com/bheisler/cargo-criterion/compare/1.0.0-alpha3...1.0.0
 [1.0.1]: https://github.com/bheisler/cargo-criterion/compare/1.0.0-alpha3...1.0.1
 [1.0.1]: https://github.com/bheisler/cargo-criterion/compare/1.0.1...1.1.0
-[Unreleased]: https://github.com/bheisler/cargo-criterion/compare/1.1.0...HEAD
+[Unreleased]: https://github.com/bheisler/cargo-criterion/compare/1.1.0...HEAD

src/config.rs

@@ -146,11 +146,13 @@ impl PlottingBackend {
 #[derive(Debug)]
 pub enum MessageFormat {
     Json,
+    OpenMetrics,
 }
 impl MessageFormat {
     fn from_str(s: &str) -> MessageFormat {
         match s {
             "json" => MessageFormat::Json,
+            "openmetrics" => MessageFormat::OpenMetrics,
             other => panic!("Unknown message format: {}", other),
         }
     }
@@ -422,10 +424,10 @@ bencher: Emulates the output format of the bencher crate and nightly-only libtes
             .arg(Arg::with_name("message-format")
                 .long("message-format")
                 .takes_value(true)
-                .possible_values(&["json"])
+                .possible_values(&["json", "openmetrics"])
                 .help("If set, machine-readable output of the requested format will be printed to stdout.")
                 .long_help(
-"Change the machine-readable output format. Possible values are [json].
+"Change the machine-readable output format. Possible values are [json, openmetrics].
 
 Machine-readable information on the benchmarks will be printed in the requested format to stdout.
 All of cargo-criterion's other output will be printed to stderr.