Re-introduce the instruments middleware

[gudmundur]

With the introduction of canonical log lines, we dropped the Pliny::Middleware::Instruments as a default middleware, see here. The side effect of that is that the start of a request is not logged. The default log lines emitted for a request are:

app=hello deployment=development request_id=c3ae2fae-0b29-4d27-9f05-402fcce7a95e count#hello.requests=1
app=hello deployment=development canonical_log_line request_id=c3ae2fae-0b29-4d27-9f05-402fcce7a95e request_ip=127.0.0.1 request_method=GET request_path=/ request_user_agent=curl/7.51.0 request_route_signature=/ response_length=6 response_status=200 timing_total_elapsed=0.033
app=hello deployment=development request_id=c3ae2fae-0b29-4d27-9f05-402fcce7a95e measure#hello.requests.latency=0.033
app=hello deployment=development request_id=c3ae2fae-0b29-4d27-9f05-402fcce7a95e count#hello.requests.status.2xx=1
app=hello deployment=development request_id=bdd42e63-005e-48b4-9d70-99698288c0e6 count#hello.requests=1

The initial count#hello.requests=1 happens at the beginning of a request but is a metrics line, and will be dropped if a different metric backend than logfmt is used.

This change brings back the Instruments middleware, although much of the information reported by it is already available in the canonical log line. It does have the benefit of indicating the start and end of a request.

[gudmundur]

@brandur What do you think about this one? Do you think we should change the Instruments middleware or just leave it as is?

[brandur]

I don't have too strong of an opinion on this one. Looking at my canonical log line patch, if I were to do this all over again, I probably would have just left the instruments middleware in as kind of a principle of least surprise thing, but I think it could go either way.

Another option might be to add in some other logger that prints out some basic information about the requests (maybe HTTP headers and the like), but that's probably overdesigning for now.

In short, if having instruments around is something that seems like it would be beneficial to you, then +1.