Where a vew is called from doesn't matter as much. We already know what
action they belong to and this is more than enough information. By
removing the file/line number from the list of tags we should also be
able to reduce the number of series stored in InfluxDB.

This gives a very rough estimate of how much memory is allocated during
a transaction. This only works reliably when using a single-threaded
application server and a Ruby implementation with a GIL as otherwise
memory allocated by other threads might skew the statistics. Sadly
there's no way around this as Ruby doesn't provide a reliable way of
gathering accurate object sizes upon allocation on a per-thread basis.

Without this it's impossible to find out what methods/views/queries are
executed by a certain controller or Sidekiq worker. While this will
increase the total number of series it should stay within reasonable
limits due to the amount of "actions" being small enough.

Every time I push to GitLab, I get > 2 emails saying a spec failed when
I don't care about benchmarks and other specs that have `allow_failure` set to `true`.

Since filtering by these values is very rare (they're mostly just
displayed as-is) we don't need to waste any index space by saving them
as tags. By storing them as values we also greatly reduce the number of
series in InfluxDB.

While useful for finding out what methods/views belong to a transaction
this might result in too much data being stored in InfluxDB.

This reverts commit 7549102b.

Apparently I was wrong about
ActiveSupport::Notifications::Event#duration returning the duration in
seconds, instead it returns it in milliseconds already.

Transaction timings are also already stored in milliseconds, this keeps
things consistent.

This ensures Rails and Sidekiq transactions are split into the series
"rails_transactions" and "sidekiq_transactions" respectively.

This removes the need for any tags to differentiate between Sidekiq and
Rails statistics while still being able to separate the two.

This makes it easier to see where time is spent without having to
aggregate all the individual points in the method_calls series.

This will be used to store/increment the total query/view rendering
timings on a per transaction basis. This in turn can greatly reduce the
amount of metrics stored.

This isn't hugely useful and mostly wastes InfluxDB space. We can re-add
this whenever needed (but only once we really need it).

This removes the need for tagging all metrics with a "process_type" tag.

This ensures we don't need to load anything from either PostgreSQL or
the Rails cache whenever creating new InfluxDB connections.

This particular setup had 3 problems:

1. Storing SQL queries as tags is very inefficient as InfluxDB ends up
   indexing every query (and they can get pretty large). Storing these
   as values instead means we can't always display the SQL as easily.
2. We already instrument ActiveRecord query methods, thus we already
   have timing information about database queries.
3. SQL obfuscation is difficult to get right and I'd rather not expose
   sensitive data by accident.

Nokogiri produces inefficient XPath expressions when given CSS
expressions such as "a.gfm". Luckily these expressions can be optimized
quite easily while still achieving the same results.

In the two cases where this optimization is applied the run time has
been reduced from around 170 ms to around 15 ms.

While it's useful to keep track of the different versions (Ruby, GitLab,
etc) doing so for every point wastes disk space and possibly also RAM
(which InfluxDB is all to eager to gobble up). If we want to see the
performance differences between different GitLab versions simply looking
at the performance since the last release date should suffice.

Prior, if we were viewing a blob at
`https://example.com/namespace/project/blob/master/doc/some-file.md` and
it contained a relative link such as `[README](../README.md)`, the
resulting link when viewing the blob would be:
`https://example.com/namespace/project/blob/README.md` which omits the
`master` ref, resulting in a 404.

This removes the need for Sidekiq and any overhead/problems introduced
by TCP. There are a few things to take into account:

1. When writing data to InfluxDB you may still get an error if the
   server becomes unavailable during the write. Because of this we're
   catching all exceptions and just ignore them (for now).
2. Writing via UDP apparently requires the timestamp to be in
   nanoseconds. Without this data either isn't written properly.
3. Due to the restrictions on UDP buffer sizes we're writing metrics one
   by one, instead of writing all of them at once.

Newlines aren't really needed and they may mess with InfluxDB's line
protocol.

This ensures we can still boot, even when the "application_settings"
table doesn't exist.

This allows us to track the counts of actual classes instead of "T_XXX"
nodes. This is only enabled on CRuby as it uses CRuby specific APIs.

This will be used to (for example) instrument all ActiveRecord models.

This ensures we don't end up wasting resources by tracking method calls
that only take a few microseconds. By default the threshold is 10
milliseconds but this can be changed using the gitlab.yml configuration
file.

This makes it possible to determine if a method should be instrumented
or not using a block.