Metrics endpoint to support charting

[dechdev]

In order to support histograms, pie charts, and other charting in Firefly-UI, an endpoint is needed to fetch an array of row counts in a table, where each index (bucket) of the array is the count for the corresponding time interval between the start and end time.

time interval = startTime - endTime / buckets

GET /api/v1/namespaces/default/metrics with the following params:
startTime: unix time stamp to specify the start time of the data you want to fetch (ex. 1638279785)
endTime: unix time stamp to specify the end time of the data you want to fetch (ex. 1638366185)
buckets: number of buckets to divide the data into between the start and end time
type: the table name of the data you want to fetch data for

Typical response times for this endpoint are 50-100ms. The buckets param is hard capped at 100 buckets to prevent expensive queries to the DB

[codecov-commenter]

Codecov Report

Merging #341 (59e4f15) into main (4682849) will not change coverage.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##              main      #341    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files          234       237     +3     
  Lines        12796     12896   +100     
==========================================
+ Hits         12796     12896   +100     

Impacted Files	Coverage Δ
internal/orchestrator/orchestrator.go	100.00% <ø> (ø)
internal/apiserver/route_get_chart_histogram.go	100.00% <100.00%> (ø)
internal/database/sqlcommon/chart_sql.go	100.00% <100.00%> (ø)
internal/orchestrator/chart.go	100.00% <100.00%> (ø)
internal/assets/token_transfer.go	100.00% <0.00%> (ø)

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[awrichar]

Looks good! Noted one small change inline on error messages.