Introduce some metrics about endpoints and their arguments/outcomes

[shepmaster]

Example metrics:

# HELP playground_request_duration_seconds Number of requests made
# TYPE playground_request_duration_seconds histogram
playground_request_duration_seconds_bucket{backtrace="false",channel="Nightly",crate_type="Binary",edition="Rust2015",endpoint="Execute",mode="Release",outcome="ErrorTimeoutSoft",target="",tests="false",le="0.1"} 0
playground_request_duration_seconds_bucket{backtrace="false",channel="Nightly",crate_type="Binary",edition="Rust2015",endpoint="Execute",mode="Release",outcome="ErrorTimeoutSoft",target="",tests="false",le="1"} 0
playground_request_duration_seconds_bucket{backtrace="false",channel="Nightly",crate_type="Binary",edition="Rust2015",endpoint="Execute",mode="Release",outcome="ErrorTimeoutSoft",target="",tests="false",le="2.5"} 0
playground_request_duration_seconds_bucket{backtrace="false",channel="Nightly",crate_type="Binary",edition="Rust2015",endpoint="Execute",mode="Release",outcome="ErrorTimeoutSoft",target="",tests="false",le="5"} 0
playground_request_duration_seconds_bucket{backtrace="false",channel="Nightly",crate_type="Binary",edition="Rust2015",endpoint="Execute",mode="Release",outcome="ErrorTimeoutSoft",target="",tests="false",le="10"} 0
playground_request_duration_seconds_bucket{backtrace="false",channel="Nightly",crate_type="Binary",edition="Rust2015",endpoint="Execute",mode="Release",outcome="ErrorTimeoutSoft",target="",tests="false",le="15"} 1
playground_request_duration_seconds_bucket{backtrace="false",channel="Nightly",crate_type="Binary",edition="Rust2015",endpoint="Execute",mode="Release",outcome="ErrorTimeoutSoft",target="",tests="false",le="+Inf"} 1
playground_request_duration_seconds_sum{backtrace="false",channel="Nightly",crate_type="Binary",edition="Rust2015",endpoint="Execute",mode="Release",outcome="ErrorTimeoutSoft",target="",tests="false"} 11.07418264
playground_request_duration_seconds_count{backtrace="false",channel="Nightly",crate_type="Binary",edition="Rust2015",endpoint="Execute",mode="Release",outcome="ErrorTimeoutSoft",target="",tests="false"} 1

This was aftter running the code

fn main() {
    println!("Hello, world!");
    std::process::Command::new("sleep").arg("100").status();
}