Metrics rename

[eh-am]

Refers to #332

Since this is a breaking change, I decided to clean up as much as possible.

• Added pyroscope_ prefix.
• Prefixed most metrics with _storage as they refer to the "storage" subsystem
• Used histogram instead of gauge for durations
• Added help messages
• Updated the dashboard. Added a "Go" row that refers to the metrics exported by the prometheus client. Even though in certain environments it may be unnecessary (let's say there's already a dashboard for go metrics), it's still useful for us as a "batteries included" dashboard. It's collapsed by default, though.

Here's the result of the promlinter tool:

$ promlinter list . --add-position --add-help 

POSITION                           TYPE                NAME                                              HELP
benchmark/main.go:208:33           GAUGE               pyroscope_benchmark                               
benchmark/main.go:212:35           GAUGE               pyroscope_run_progress                            
benchmark/main.go:74:48            COUNTER             pyroscope_upload_errors                           
benchmark/main.go:78:53            COUNTER             pyroscope_successful_uploads                      
pkg/storage/storage.go:115:53      COUNTER             pyroscope_storage_writes_total                    number of calls to storage.Put
pkg/storage/storage.go:119:52      COUNTER             pyroscope_storage_reads_total                     number of calls to storage.Get
pkg/storage/storage.go:125:51      HISTOGRAM           pyroscope_storage_evictions_duration_seconds      duration of evictions (triggered when there's memory pressure)
pkg/storage/storage.go:132:52      GAUGE               pyroscope_storage_evictions_alloc_bytes           number of bytes allocated in the heap
pkg/storage/storage.go:136:52      GAUGE               pyroscope_storage_evictions_total_mem_bytes       total number of memory bytes
pkg/storage/storage.go:141:60      HISTOGRAM           pyroscope_storage_caches_flush_duration_seconds   duration of storage caches flush (triggered when server is closing)
pkg/storage/storage.go:146:60      HISTOGRAM           pyroscope_storage_db_close_duration_seconds       duration of db close (triggered when server is closing)
pkg/storage/storage.go:152:51      HISTOGRAM           pyroscope_storage_writeback_duration_seconds      duration of write-back writes (triggered periodically)
pkg/storage/storage.go:157:51      HISTOGRAM           pyroscope_storage_retention_duration_seconds      duration of old data deletion
pkg/storage/storage.go:192:56      COUNTER             pyroscope_storage_cache_hits_total                total number of cache hits
pkg/storage/storage.go:196:57      COUNTER             pyroscope_storage_cache_misses_total              total number of cache misses
pkg/storage/storage.go:200:64      COUNTER             pyroscope_storage_cache_reads_total               total number of cache queries
pkg/storage/storage.go:204:65      COUNTER             pyroscope_storage_cache_persisted_total           number of items persisted from cache to disk
pkg/util/debug/reporter.go:37:48   GAUGE               pyroscope_storage_disk_bytes                      size of items in disk
pkg/util/debug/reporter.go:41:53   GAUGE               pyroscope_storage_cache_size                      number of items in cache (memory)
pkg/util/debug/reporter.go:53:47   GAUGE               pyroscope_cpu_utilization                         cpu utilization (percentage)

[codecov]

Codecov Report

Merging #346 (7c0056e) into main (f3186f4) will decrease coverage by 0.10%.
The diff coverage is 76.67%.

@@            Coverage Diff             @@
##             main     #346      +/-   ##
==========================================
- Coverage   54.49%   54.40%   -0.09%     
==========================================
  Files         102      102              
  Lines        5020     4988      -32     
==========================================
- Hits         2735     2713      -22     
+ Misses       2035     2025      -10     
  Partials      250      250              

Impacted Files	Coverage Δ
pkg/util/debug/reporter.go	0.00% <0.00%> (ø)
pkg/storage/periodic.go	75.48% <66.67%> (+0.04%)	⬆️
pkg/storage/cache/cache.go	56.87% <100.00%> (+0.99%)	⬆️
pkg/storage/storage.go	65.21% <100.00%> (-1.53%)	⬇️

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[petethepig]

Looks great, merging